The development of hypothalamic control of PRL secretion in the late prenatal and early postnatal periods in the rat was studied by employing a static system for incubation of pituitaries. PRL was detected in the incubation medium after incubating fetal pituitaries as early as day 18. A gradual increase in the amount of hormone released into the medium occurred during development, with the greatest change occurring between days 20 and 21 prenatally. A progressive increase in the pituitary content of PRL occurred during development. The percentage of PRL released from the gland was higher pre- (19-39%) than postnatally (17%). Hypothalamic extracts from fetuses and neonates stimulated the secretion of PRL when coincubated with pituitaries from animals of the same age. However, extracts from 1-day-old neonates inhibited PRL secretion from adult male hemipituitaries. Extracts prepared from adult male hypothalami stimulated PRL secretion from neonatal pituitaries from rats at 1 day of age, as did cerebral cortical extracts from adults, but not 1-day-old, rats. TRH significantly stimulated PRL secretion in vitro from pituitaries of donors as early as day 19 of the fetal period. The response of the pituitaries to this peptide diminished by day 8 postnatally. Immunoneutralization of hypothalamic TRH significantly decreased but did not abolish the PRL-releasing activity of hypothalamic extracts, whereas this procedure had no effect on the GH-releasing activity of the extracts. The dopamine receptor agonist apomorphine (10(-5) M) inhibited PRL secretion in vitro on day 19 of the fetal period and postnatally starting on day 1. The response to apomorphine increased with advancing age. The results suggest that a combination of factors contribute to maintaining high circulating PRL levels during the late fetal and early neonatal periods. These include high rates of PRL release by the pituitary and a relative insensitivity of the pituitary to dopamine in the face of high sensitivity to PRL-releasing factors such as TRH.