The role of the neurotransmitter serotonin (SER) in regulating the hypothalamic-pituitary function is not completely clarified, although considerable evidence suggests a dominant stimulatory control of ACTH and PRL and a dominant inhibition of TSH, with more variable effects on GH secretion. Furthermore, SER has been implicated in the regulation of appetite, given the anorectic activity of SERergic drugs in animals and humans and experimental evidence suggesting that animal obesity can be a function of SER depletion. In order to study the hypothesized impairment of the SERergic system in human obesity, PRL, GH, TSH and cortisol levels before and after oral administration of the SER precursor 5-hydroxytryptophan (OH-TRY) (500 mg), were determined in 10 obese (BMI 41.5 +/- 1.68 kg/m2) but otherwise healthy women (mean age 50.9 +/- 1.43 yr) and in control group of 7 normal-weight (BMI 20.9 +/- 0.66 kg/m2) women (mean age 49.8 +/- 1.18 yf) after oral administration of the SER precursor 5-hydroxytryptophan (OH-TRY) (500 mg). The results were matched against placebo. In contrast with placebo, OH-TRY administration provoked a PRL and cortisol increase in all the subjects: PRL levels increased in controls at 120 min (p < 0.05) and at 180 min (p < 0.01) and in obese women at 120 and 180 min (p < 0.01); cortisol levels increased in both groups at 90, 120, 180 min (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)