Hypothalamic-pituitary-ovarian function in women during the first month postpartum has been studied. The plasma LH-RH level in puerperal women was very low on days 5 and 10 postpartum and thereafter increased, but was still significantly lower on day 30 postpartum than that in eumenorrheic women on day 8 or 9 of the normal cycle. Serum FSH reached a very low level on days 5 and 11 postpartum, then gradually increased to a slightly higher level on day 30 postpartum than that in eumenorrheic women. The serum estradiol level fell immediately after delivery and remained low until day 30 postpartum. No response of serum FSH or LH to LH-RH was found on day 11 postpartum. However, daily injections of LH-RH from days 5 through 10 postpartum caused an increased secretion in FSH on day 11 postpartum, both basally and in response to LH-RH. On day 20 postpartum, serum FSH responded excessively to LH-RH in all women, but the response in serum LH was restored in a half of them. The mean basal level of serum FSH was significantly higher in cases demonstrating the response in serum FSH and LH than that in cases demonstrating the response in only serum FSH. Serum FSH and LH showed excess and normal responses respectively to LH-RH on day 30 postpartum. It is concluded that hypothalamic secretion of LH-RH decreases during the first month postpartum, especially for the first 10 days, and the low basal level of serum FSH and unresponsiveness of the pituitary to LH-RH observed in the early puerperium may result from hypothalamic hypofunction.