Serum prolactin (PRL) levels in puerperium were examined in order to determine the effect of prolactin on the recovery of the Hypothalamo-Pituitary-Ovarian Axis. It required thirteen weeks for the lactating group, four weeks for the non-lactating group and two days for the group receiving bromocriptine for serum PRL to be restored to pre-pregnancy levels. There was no significant difference between the lactating and non-lactating groups in FSH and LH levels. E2 tended to show higher levels in the non-lactating group until the fifteenth postpartum week. In the group receiving bromocriptine, the LH and FSH levels were significantly higher during the second postpartum week as compared with the lactating group. The LH-RH test was positive for the bromocriptine group. The mean periods between delivery and reappearance of the first ovulation were 143 days in the lactating group, 82 days in the non-lactating group and 55 days in the bromocriptine therapy group. Patients in whom PRL returned quickly to the normal level also showed early resumption of ovulation. From these results it can be postulated that elevated basal PRL levels in the puerperium inhibit the Hypothalamo-Pituitary Axis, producing a delay in the resumption of ovulation. Elevated basal PRL levels also had an inhibitory effect on ovarian response to gonadotropin.