To fall in love is the first step in pair formation in humans and is a complex process which only recently has become the object of neuroscientific investigation. The little information available in this field prompted us to measure the levels of some pituitary, adrenal and gonadal hormones in a group of 24 subjects of both sexes who had recently (within the previous six months) fallen in love, and to compare them with those of 24 subjects who were single or were part of a long-lasting relationship. The following hormones were evaluated by means of standard techniques: FSH, LH, estradiol, progesterone, dehydroepiandrosterone sulphate (DHEAS), cortisol, testosterone and androstenedione. The results showed that estradiol, progesterone, DHEAS and androstenedione levels did not differ between the groups and were within the normal ranges. Cortisol levels were significantly higher amongst those subjects who had recently fallen in love, as compared with those who had not. FSH and testosterone levels were lower in men in love, while women of the same group presented higher testosterone levels. All hormonal differences were eliminated when the subjects were re-tested from 12 to 24 months later. The increased cortisol and low FSH levels are suggestive of the "stressful" and arousing conditions associated with the initiation of a social contact. The changes of testosterone concentrations, which varied in opposite directions in the two sexes, may reflect changes in behavioural and/or temperamental traits which have yet to be clarified. In conclusion, the findings of the present study would indicate that to fall in love provokes transient hormonal changes some of which seem to be specific to each sex.