The release of serotonin (5-HT) from nerve terminals located within the medio-basal hypothalamus has been shown to interfere with the secretion of pituitary gonadotropins. In particular, it inhibits the cyclic surge of LH resulting in ovulation, and it facilitates the suckling-induced rise in plasma prolactin levels in lactating rats. The level of activity of 5-HT neurons also seems to affect the uptake of -3Hestradiol by the hypothalamus. The importance of 5-HT in the neuroendocrine control of gonadotropic secretion is discussed in the light of these findings.