It has been previously shown that the double PRLRKO/ D2RKO mice present more marked pituitary hyperplasia and increased pituitary weight compared to the single D2RKO counterpart. Our results may explain why abrogation of PRLR signaling in PRLKO or PRLRKO mice contributes to pituitary hyperplasia. PRLRKO female mice develop pituitary hyperplasia from 6 months of age, whereas male mice do not show pituitary enlargement until 18 months of age. In females, the apoptotic and proliferative rates are acutely and closely regulated at each stage of the estrous cycle. Our results indicate that in PRLRKO female mice there is a progressive and accumulative effect due to the lack of PRL apoptotic and antiproliferative actions in each proestrus. These results also could explain why PRLRKO female mice develop pituitary hyperplasia earlier than males, although PRL also plays a key role in regulating pituitary remodeling in males. Based on the evidence provided here that PRL, like dopamine, regulates anterior pituitary homeostasis by controlling cell proliferation and apoptosis, our findings further support the earlier hypothesis of Schuff et al that a deficiency of these two individual pathways results in additive effects on the development of pituitary tumors in double PRLRKO/D2RKO mice. In accordance with this hypothesis, we also observed that chronic blockade of PRLR by the antagonist D1-9-G129R-hPRL increased the pituitary weight in TG female mice. Based on these results, we propose a general model depicted in Figure 8. At proestrus, the high circulating levels of estrogens upregulate PRL secretion which in turn, together with estradiol itself, induces apoptosis of anterior pituitary cells. Furthermore, PRL stimulates dopamine release from hypothalamic dopaminergic neurons to the portal vessels. Dopamine reaching the anterior pituitary could also contribute to the apoptosis observed in the gland at this stage of the estrous cycle. Estradiol also increases anterior pituitary PRL cleavage into 16 kDa N-terminal fragments, which in turn induce apoptosis in an estrogen-dependent manner, by a PRLRindependent mechanism. The molecular mechanisms by which PRL regulates pituitary homeostasis during the estrous cycle are unknown. Since different PRLR isoforms exhibit different capabilities to activate intracellular signaling pathways, the ultimate effects of PRL depend not only on the number, but also on the type of receptor isoforms expressed in target tissues. It is thus important to determine the pattern of PRLR expression before moving further in intracellular signaling investigations. In the present study we show that the anterior pituitary from female mice expresses short and long PRLR isoforms and that expression of both are increased at proestrus. This suggests that PRLR expression in the anterior pituitary is regulated by circulating hormones whose levels fluctuate during the estrous cycle.