Neonatal exposure to testicular androgens interferes with the ability in adulthood to release an ovulatory amount of LH, with the display of female sexual behavior, and also with control mechanisms of the activity of the corpora lutea. It is likely that effects on luteal activity involve, in part, effects on the control of PRL secretion. Therefore, serum concentrations of PRL were studied in ovarian graft-bearing male rats, castrated as neonates (NC-males) or as adults (AC-males), during a luteal phase induced and temporarily sustained by an ectopic pituitary graft. Before the removal of the pituitary graft, serum PRL and progesterone levels were high at all times when measured during the day in both AC-males and NC-males. Removal of the pituitary graft, 6-7 days after ovulation led to immediate cessation of luteal activity in AC-males but not in NC-males. In the latter animals, luteal activity was maintained for at least 6 days after pituitary graft removal by intermittently high levels of PRL, i.e. around 0400 and 1900 h. However, in AC-males, PRL levels were generally low after pituitary graft removal. It is concluded that the absence of neonatal exposure to testicular secretion contributes to the ability of the adult rat to release PRL in a diurnal surge-like manner. The inability to release PRL in this way may explain the failure of AC-males to maintain luteal activity.