The effects of serotonin on PRL and LH release were investigated in female and male rats under different experimental conditions. At a dose of 5 mg/kg ip, serotonin increased serum PRL titers in intact males and in females during diestrus and estrus; the levels attained in the male rats were much higher than in the females. At a lower dose (2.5 mg/kg) the PRL-releasing effect of serotonin was only evident in male rats. Thus, we chose this dose for the following experiments to investigate the apparent sexual difference. To evaluate the importance of the hormonal status characteristic of male and female in conditioning the serotonin effect, an experiment was performed in gonadectomized rats, untreated or treated with estradiol benzoate (EB), or testosterone propionate (TP). In the three hormonal conditions the sexual difference was maintained: serotonin released PRL in males and failed to do so in females. However, if males were castrated within 24 h of birth, and females androgenized by a single perinatal injection of TP, the sexual difference in adulthood were reversed; thus, androgenized females responded to serotonin and males castrated at birth failed to do so. These results suggest that a male differentiated brain is more sensitive to the PRL-releasing effect of serotonin, irrespective of the hormonal environment of the rat. On the other hand, serotonin increased serum LH in female rats in estrus and in adult ovariectomized rats treated with EB; but not in females in diestrus or in ovariectomized rats, treated with TP or untreated. Neither did it modify serum LH titers in male rats whether intact, orchidectomized, or orchidectomized plus steroids. However, if male rats were castrated a few hours after birth and then treated in adulthood with EB, serotonin effectively released LH. Thus, two components, estradiol and a feminine differentiated brain, may be necessary for the facilitatory action of serotonin on LH release. Since no sex differences were observed in the increase of serum serotonin after the injection of 2.5 mg/kg of the drug, it can be discounted that the differences described for the endocrine effect of the drug could be due to different levels of circulating indolamine achieved in male and female rats. Taken together, our results indicate that serotonergic control of anterior pituitary secretion is sexually differentiated and that it presents individual characteristics for PRL and LH release.