Pseudopregnancy (PP) in the rat is characterized by maintenance of the corpora lutea and twice daily surges of prolactin at 1500-2100 h and 0100-0900 h. Four days after PP was induced by cervical stimulation with a glass rod, rats were cannulated via the right carotid artery and one day later received an implant of ovine prolactin (200-250 mg) or albumin (controls) into the median eminence of the hypothalamus. Forty-five of 54 rats implanted with prolactin showed a termination of PP within 4 days. Ova were recovered from 19 of these rats whereas all of the controls remained pseudopregnant and did not ovulate. Ovarian and uterine weights were increased in rats implanted with prolactin. Plasma prolactin levels in the rats implanted with prolactin were lower each day (45-89%) at 0100, 0300 and 1800 h compared to controls. Plasma LH levels rose gradually after prolactin implantation, and in those rats in which ovulation occurred, a preovulatory LH rise was detected at 1800 h on the previous day. It is concluded that daily surges of prolactin play an essential role in maintaining PP in the rat, and that implantation of prolactin into the median eminence results in inhibition of prolactin. The stimulation of LH secretion, which led to ovulation, may be the result of a direct neural action of prolactin on the LH release mechanism or may simply result from the termination of PP.