5-hydroxytryptophan (5-HTP), a serotonin precursor, has been shown to induce LH release in female but not in male rats at 16, 18 and 20 days of age. The purpose of this study was to investigate the role of neonatal gonadal hormones on 5-HTP-induced LH release in male and female rats of 16 and 20 days of age. The following groups of rats were studied (a) control females, (b) androgenized females, (c) control males, and (d) castrated males. Androgenization and castration were performed within 48 h of birth. 5-HTP administration increased LH concentration in normal females and castrated males at 16 and 20 days of age. The LH release response to 5-HTP was significantly lower at 20 than at 16 days of age in both groups. Neonatal androgenization abolished the LH release response in prepubertal female rats. These results indicate that neonatal exposure to androgen is responsible for the sex difference in the LH response to 5-HTP, which may be associated with a regulatory action of serotonin on the cyclic hypothalamic control of gonadotrophin secretion.