Animals bearing intrahypothalamic anterior pituitary (AP) grafts exhibit a central hyperprolactinemia, and thus, serve as a model for the study of short-loop feedback regulation of prolactin (PRL) secretion. We investigated the effects of intrahypothalamic AP grafts on PRL secretion during late pregnancy (n = 7). A further group of five rats was injected with the dopamine against bromocriptine during late pregnancy and five control rats were treated with the bromocriptine-vehicle only. A nocturnal surge in plasma PRL concentrations was observed in the vehicle-injected control animals, peaking at 212 +/- 11 ng/ml at 0300 h on the day of parturition. Despite the central hyperprolactinemia due to the grafts, a similar PRL surge was observed in grafted animals, peaking at 205 +/- 35 ng/ml at 0300 h on the day of parturition. Bromocriptine treatment completely blocked the nocturnal surge of PRL. These results suggest that short-loop feedback autoregulation of PRL secretion becomes less responsive or nonfunctional in the last 24 h of pregnancy in the rat. This apparent change in sensitivity of the autofeedback mechanism may be an important physiological mechanism to allow the hypersecretion of PRL during lactation.