Serum levels of prolactin (PRL), FSH [follicle stimulating hormone] LH [luteinizing hormone] and estradiol-17.beta. were determined by radioimmunoassay in 57 lactating women and in 20 women in whom lactation was inhibited by ergocryptine (CB-154). Women who breast fed their infants exhibited high PRL levels which abruptly declined within 48 h postpartum, and remained low for the duration of the study. Serum FSH was undetectable during the 1st wk postpartum in lactating and in CB-154 treated women. Thereafter, lactating women showed increasing FSH levels which reached a maximum by the 3rd wk postpartum. These FSH values were higher in lactating women than in the CBS-154 treated group. In contrast, LH levels were higher in those women receiving CB-154. Serum estradiol-17.beta. remained at low levels throughout the study, and no difference was observed between the 2 groups of subjects. Inhibition of PRL secretion apparently leads to a faster recovery of gonadotropin secretion toward the menstrual type, and PRL suppression produces no effect on the ovarian estrogen production.