In the last two decades, opioid analgesics have assumed an important place in general anesthetic practice in the United States. Part of the reason for this has been the introduction of the potent new agonists fentanyl, sufentanil, and alfentanil. Because of problems with morphine-oxygen anesthesia (incomplete amnesia, occasional histamine-related reaction, marked increases in intra- and postoperative respiratory depression), a suitable alternative was sought but not found among existing opioids. A breakthrough came in 1960, when fentanyl was synthesized, laying the foundation for a better understanding of the structure-activity relationships of narcotic analgesics and stimulating interest in developing compounds with even greater potency and safety margins. Investigators interested in opioid anesthesia began to study fentanyl in animals and then in humans. Fentanyl (50-100 micrograms/kg) with oxygen (100%) was evaluated as an anesthetic in patients undergoing mitral valve and coronary artery surgery. Changes in cardiovascular dynamics with induction doses ranging from 8 to 30 micrograms/kg consisted of small decreases in heart rate and arterial blood pressure. All other cardiovascular variables studied, including cardiac output, remained unchanged, even with additional doses up to 100 micrograms/kg. It was determined that fentanyl had use as a narcotic anesthetic, despite its potential for cardiovascular depression and stimulation, respiratory depression, muscle rigidity, and, occasionally, incomplete anesthesia. Since the introduction of fentanyl, two other potent synthetic opioids have been introduced into clinical practice--sufentanil and alfentanil.