Serum prolactin, progesterone and rat placental lactogen (rPL) were measured in pregnant rats following removal of various numbers of conceptuses and their placentas on Day 8 of pregnancy. Blood samples were taken during the time of the expected nocturnal surge of prolactin on Days 8 and 9, and at the same time on Days 11 and 14 of pregnancy, when the surges are normally no longer present. As the number of conceptuses present decreased, the number of days the prolactin surge was present increased. Measurement of the early form of rPL (rPL-I) by lymphoma cell bioassay revealed a proportionate decrease in serum rPL-I levels on Days 9 and 11 as the number of conceptuses was decreased. This also was true for the late form of rPL (rPL-II) when measured by RIA on Day 14 of pregnancy. Thus, as rPL levels are reduced, prolactin surges remain present for increased numbers of days. This suggests that rPL may normally be an inhibitory factor to prolactin secretion during pregnancy, and is responsible for the termination of the prolactin surges at midpregnancy.