To summarize luteal function during pregnancy briefly, there are physiological processes initiated by the embryo and/or conceptus in early pregnancy that serve to prolong the life-span of the corpus luteum. Some of these processes are well defined, but others remain more obscure. The corpus luteum is maintained in a functional state throughout pregnancy (at least in those species described in this review), even though in several species progesterone production by the corpus luteum is not required after the first third of the gestational period. The cessation of secretory function by the corpus luteum of pregnancy at the end of gestation is apparently actively induced. There is evidence in some species (especially the goat) that this is due to PGF2alpha released from the uterus or placenta. It is concluded that the occurrence of luteal regression in several species of mammal can be attributed to the physiological release of PGF2alpha from both the pregnant and nonpregnant uterus.