In a frantic, rambling letter filled with desperation, Ted Kaczynski pleaded for his brother's help in severing all ties with his family. Kaczynski had come to blame his parents for raising him to be a social misfit so fearful of rejection that he sought refuge in the Montana wilderness for nearly a quarter-century:

"It is a matter of life and death, and this is not an exaggeration," he declared in a handwritten screed to his brother, David, in the summer of 1991. "I seriously believe I will die if you can't accomplish this for me. . . . I won't be able to eat or sleep or stop my heart from pounding until this whole thing is settled."

"I have got to know, I have GOT TO, GOT TO, GOT TO know that every last tie joining me to this stinking family has been cut FOREVER and that I will never NEVER have to communicate with any of you again. . . . I've got to do it NOW. I can't tell you how desperate I am. . . . It is killing me," the 10-page letter said.

There were hundreds of missives Theodore J. Kaczynski sent to his family during the years he lived in a primitive cabin near the Continental Divide until he was arrested last April as the Unabomber suspect. The letters, never before disclosed, provide a penetrating look at his bizarre self-imposed solitude, a farrago of musings and rantings that reveal a brilliant but irrational soul whose hold on reality is fragile at best.

It is a chilling portrait of a man who lived in fear of social rejection, who was desperate for a girlfriend, who was tormented by violent dreams, who blamed his mother for his plight. Scrawled by pen, crammed with a blizzard of words and some drawings, the writings have become a central part of a campaign by the Kaczynski family to persuade prosecutors not to seek the death penalty.

The 54-year-old, Harvard-educated mathematician is suspected of carrying out a 17-year, coast-to-coast bombing spree in which three people were killed and 23 injured, a reign of terror that was the focus of the longest manhunt in U.S. history. Grand juries in California and New Jersey have indicted Kaczynski on charges relating to the bombing deaths, as well as several other attacks.

With the defendant confined to a Sacramento prison cell, Attorney General Janet Reno has received input from local prosecutors, victims' relatives and Kaczynski's family in weighing the capital punishment question.

The family, through Washington attorney Anthony P. Bisceglie, has petitioned Reno to spare Kaczynski's life partly because they believe he is mentally ill, arguing that under U.S. law this is a "mitigating factor" against capital punishment. In an interview with The Washington Post last week, David Kaczynski contended that the letters from his brother, turned over to federal investigators, capture how disturbed he is.

"Through the years, the letters have shown sudden and unpredictable mood swings, a preoccupation with disease, extreme phobias, compulsive thinking and an inability to let go of minutiae," David Kaczynski said. "One senses a psyche that feels itself terribly isolated and threatened in the world, tormented by its own complexities, unable to hold things in their proper perspective or to find comfort, security or rest for itself."

Justice Department lawyers have been struggling with the death penalty issue because of questions surrounding Kaczynski's mental health. The family has also urged Reno to reject the death penalty out of fairness because David Kaczynski was crucial to cracking the case. It was after reading the Unabomber's anti-technology "manifesto," published by The Washington Post and the New York Times in 1995, that he recognized aspects of his brother's thinking and writing.

"Our interest from the beginning was to protect life, and if this government were to process this like a cold and calculative machine, I would have to conclude my faith in that system was misplaced," David Kaczynski said. "What would a future family member in a similar situation think if I were repaid with my brother's death? It would be the ultimate disincentive for anyone else to cooperate with our justice system." Fear of Rejection

In the years that Ted Kaczynski lived a mountain man's existence, writing letters became his sole means of communicating with his family, except for occasional visits from his parents and brother, which he eventually prohibited.

One of his more alarming missives, in the summer of 1991, was sent to his mother. He wrote that his early life had been shaped by traumatic social experiences that had left him profoundly wary of others.

"Suppose that for a period of years whenever you touched -- let us say -- a banana, you got a severe electric shock. After that you would always be nervous around bananas, even if you knew they weren't wired to shock you," he said. "Well, in the same way, the many rejections, humiliations and other painful influence {sic} that I underwent during adolescence at home, in high school, and at Harvard have conditioned me to be afraid of people."

Kaczynski revealed that he is "always under stress" whenever he is around people, except those he has known for a long time. The reason is that he doesn't feel that people will accept him.

"This fear of rejection -- based on bitter experience both at home and at school -- has ruined my life, except for the few years that I spent alone in the woods, largely out of contact with people," he wrote.

While he has not made a friend during his entire adult life, Kaczynski noted, he nonetheless finds solitude "congenial." There are regrets, however -- deep ones. Male friendships he can do without, but female companionship is another matter.

"Women are gentle, nice, pleasant to be with, they represent warmth, joy, family life, love and, of course, sex. Naturally, women have their faults too and moreover not all women have the good qualities I've just mentioned," he wrote. "But for 37 years I've desired women. I've wanted desperately to find a girlfriend or a wife but have never been able to make any progress toward doing so because I lack the necessary social self-confidence and social skills." He attributed his vivid ruminations about women to having made the mistake of going to a female doctor in town.

Kaczynski said he was 49 years old and would be an "old man" in a few years, one with no wife, children or any friends to speak of, and nothing to look forward to but old age and death. "I am tormented by bitter regret at never having had the opportunity to experience the love of a woman," he wrote.

The letter closed by accusing his mother of not nurturing in him the social skills that would have enabled him to relate to people. He wrote coldly that he will hate her forever "because the harm you did me can never be undone."

The family was confounded by another passage in which Kaczynski claimed that rejection he suffered at home and in school took a physical toll on him by stunting his growth. This, he believed, was the likely reason he was three inches shorter than his brother.

A dominant theme in the letters, beginning around the mid-1970s, is Kaczynski's belief that his parents were insensitive, if not cruel, during his upbringing. This was the root of his reclusive manner, he insisted, not possible mental illness, as his family had suggested. He seemed to scoff at his mother's suggestion that a prolonged and traumatic hospital stay as an infant, in which he was separated from her, might have contributed to his anger and pain.

"OK, now let's take your contention that because I was a gloomy' etc. kid, the parents had reason to believe I really was sick' . . . Let's go even further and assume I was a real nuthouse case -- let's suppose I went around insisting that I was Napoleon Bonaparte," Kaczynski wrote in an April 1986 letter to his brother. "Far from justifying our parents' behavior, that makes it even worse. They certainly knew enough to realize that if someone really is mentally ill, one of the worst things you can do to them is to shout at them in a hostile and accusing manner, You're sick! You're sick! You have the mind of a two-year old!' "

He added: "Neither you nor they seem to be able to get this obvious point through your thick skulls. They (and now you) keep citing supposedly sick' symptoms of mine in order to justify their behavior toward me. The only way to explain this is by assuming that they (and you) are more anxious to justify themselves than they are to get to the heart of the matter," the letter said. "I flatly refuse to accept any contradictions on this point. No doubt this is unreasonable. But you're just going to have to humor me if you want to get along with me."

David Kaczynski said during the two-hour interview that Ted's recollections about his parents were far-fetched. They seemed to be distortions, which sometimes plagued his memory, "amplifications or exaggerations of what happened that seem quite unreasonable, though they seem quite real to him," he said.

He noted that his brother became extremely defensive at suggestions that he might have mental problems, and threatened to break off communication. "Since I'm still mad, don't write to me for a while. Permission to send me a book for my birthday is rescinded," Ted instructed in one letter.

"It was as if he had created a system of interpretation so that there was nothing you could do to get him to look at something differently," David Kaczynski said. "Nothing could alter his way of looking at the world."

David got the impression that Ted was projecting his own mental problems by diagnosing one of David's friends as a likely candidate for schizophrenia.

"I agree there is no clear cut line dividing insanity from sanity, and that mental illness' often is a mere label pinned on those who don't act as society demands," Ted Kaczynski wrote. "Further I would question whether mental illness' and insanity' are even useful concepts. . . . On the other hand, when someone is tormented by strange visions and disagreeable feelings that pass through his head . . . it seems absurd to refrain from calling his condition a disease." A Distressing Dream

In a 10-page letter in the mid-1980s, Kaczynski recounted a dream that began with three members of a cult group appearing at the family's Evergreen Park, Ill., home.

Their goal was to tighten their control over David, but Ted wouldn't let them. He killed the trio; the last of the threesome "he tore to pieces" with his bare hands. The cult's leader, however, was still to come.

He appeared as a short, portly, middle-aged man with a pleasant demeanor, but Kaczynski sensed something sinister about him. The man introduced himself as "Lord Daddy Lombrosis" before turning into a tall, handsome man with a paternal, dignified appearance. "I felt awed by him and thought This is God!' " Kaczynski wrote. "Yet in my heart I defied him." He felt the man was trying to gain psychological control over him through "some sort of deception."

"Gradually, the room became dark and his face turned into a television screen; the pupils of his eyes became two black dots that flew around on the television screen in symmetrical patterns. But still I defied him and stood between him and you."

The room became light again, the television vanished and the man reappeared. As he walked out of the house, becoming invisible and leaving only footprints in the snow, Kaczynski had a powerful feeling that "ALL IN THAT HOUSE WERE TO BE LEFT WITHOUT HOPE." Kaczynski ran after him "begging him not to leave like this, not to leave my little brother without hope."

And then he awoke, gripped by a terrible sense of fear and foreboding. He concluded that "Lord Daddy Lombrosis stood for the Technological Society itself."

Before closing the letter, Kaczynski told his brother that he had no further desire to communicate with him, other than exchanging Christmas greetings and occasional notes.

"You're a fool," he wrote. "Go to Hell. (But I say that affectionately.)"

Over the years, Kaczynski's feelings for his brother seemed to vacillate. In one letter, he told David he had contempt and disgust for him and often found him repulsive, while in another missive he explained that seeing him enjoy life brought him comfort.

Kaczynski's feelings for his parents also seemed erratic, swinging from visceral resentment to apologetic gratitude. In a May 1970 letter, he called them the best parents that anybody could ask for and said he hoped they would forgive him for disappointing them. It was one of the only letters in which he used "love" in the closing.

But years later, Ted again crackled with contempt for them, saying how satisfied he was that his mother and father had finally conceded they were failures as parents. "The resentment I have toward you will always remain, but your last letter does soften my attitude a little," he wrote. "Enough anyway, so that I will take back what I said about hoping you drop dead on Christmas -- cause it's true that you were always good to me on Christmas."

But in a subsequent letter to David in March 1986, Kaczynski questioned the sincerity of his parents' apology. "(Isn't there a hint there of something like, we are truly sorry you turned out so rotten'?) . . . It was cold and curt and afterward she seemed to just shove it under the carpet and forget about it."

From his Montana cabin, Kaczynski increasingly warned his family not to irritate him: "You can write to me whenever you like, but please DON'T try to psychoanalyze me, and TRY not to get me upset."

David Kaczynski recalled: "Progressively, more and more subjects were off-limits because they upset him so much to talk about. I came to sense over time that he literally could not stop thinking or obsessing about some things; it would interfere with his sleep or heartbeat. Many of these subjects could plunge him into deep distress." Obsession With Details Ted Kaczynski's preoccupation with details can be seen in the letter in which he finally gave David permission to ship him a book for his birthday, provided its width did not exceed seven inches, which would require a trip to the post office. Also, the book had to be sent with the understanding that Ted could trade it in for another book.

"You get the sense of how deeply he thinks about details and has to anticipate every contingency in advance," David Kaczynski observed. "It's as if he can't take things as they come."

In a September 1985 letter, Kaczynski had difficulty setting a firm date to visit David, who was living in Texas. He chose Jan. 25 as an arrival date, but listed two alternative dates on which David should pick him up at the bus station in case of travel difficulties. On the day David was to meet his brother, however, he received a letter from Ted saying that he was scrapping the trip.

"I always connected his apprehension about traveling with his uneasiness around other people, that the social world was not a safe world for him, like it was loud music he could not shut out," David said.

The last letter the family received from Ted Kaczynski was in November 1995. He refused the offer of a visit from David, who by now suspected that his brother might be the Unabomber.

"I am not suffering, sick or discouraged,' and I don't know what indications' you think you have that I am so. But if you want me to get sick, all you have to do is keep trying to communicate with me, because I get just choked with frustration at my inability to get our stinking family off my back once and for all, and stinking family' emphatically includes you. "So get this straight . . . I DON'T EVER WANT TO SEE YOU OR HEAR FROM YOU, OR ANY OTHER MEMBER OF OUR FAMILY, AGAIN." CAPTION: David Kaczynski CAPTION: Theodore J. Kaczynski