In most eutherian species the function of the corpus luteum (CL) is influenced by extrinsic factors and it is subordinate to the pituitary, placenta, or uterus. In contrast, in marsupials the CL is relatively autonomous. Although the pituitary is essential for the formation of the CL, thereafter the secretory activity of the CL is independent of luteotrophic support, and the uterus is not luteolytic. Furthermore, the life span of the CL is unaffected by pregnancy, except in the Macropodidae (kangaroos and wallabies), in which the secretory activity of the CL is shortened under the influence of the fetus. At parturition the macropodid fetus, possibly via a release of glucocorticoids, causes the release of prostaglandins, presumed to be of uterine origin. The effect of the prostaglandin is to induce the release of prolactin from the maternal pituitary. Prolactin, and not prostaglandin, induces luteolysis and advances the events of post-partum oestrus. In the non-pregnant cycle, the mechanism of luteolysis is different; it does not involve prolactin, and the luteolytic signal is of non-uterine, possibly intrinsic, origin.