The present studies examined whether the pituitary hormones, particularly prolactin, have any role in the regulation of rat placental lactogen (rPL) secretion. Total rPL was measured using a lymphoma cell bioassay. Hypophysectomy on day 13 of pregnancy increased total serum rPL levels above those of intact control rats and delayed for 3 days the decline in rPL usually seen by day 14. To test the effect of early hypophysectomy on rPL secretion, a delayed implantation model was used. Hypophysectomy was carried out on day 3, progesterone (2 mg) was given daily until day 8 and oestrone (1·0 μg) was given on day 8. This initiated implantation on day 9, which is 4 days later than normal. Rats hypophysectomized on day 3 had high serum levels of rPL (10–13 mg/l) 7 days after initiation of implantation compared with control values of 2–3 mg/l, and these levels remained raised for the duration of the experiment. Termination of maintenance injections of steroids did not affect the high levels of rPL for several days, even though there was fetal and placental resorption. When a pair of anterior pituitaries was transplanted under the kidney capsule on day 13 (day 9 of development) of rats hypophysectomized on day 3, serum rPL levels still increased for the next 3 days. However, unlike similarly treated rats not receiving transplants, rPL levels fell rapidly thereafter and were only 5% of those in control animals 1 week later. Only the intact rats and those hypophysectomized either on day 3 or 13 and given daily maintenance doses of steroids had normal pregnancies; the other two groups in which steroid injections were stopped on day 15 showed various degrees of resorption or abortion. It is suggested that the pituitary, possibly because of its prolactin secretion, is important in the regulation of rPL secretion.