Transgenic female mice overexpressing the hCGβ subunit (hCGβ(+)) and producing elevated levels of luteinizing hormone (LH)/hCG bioactivity present as young adults with enhanced ovarian steroidogenesis, precocious puberty, and infertility. They subsequently develop pituitary prolactinomas, high circulating prolactin (PRL) levels, and marked mammary gland lobuloalveolar development followed by adenocarcinomas. None of these phenotypes appear in gonadectomized mice, indicating that the hCG-induced aberrations of ovarian function are responsible for the extragonadal phenotypes. PRL receptor-deficient (PRLR(-/-)) female mice are sterile, despite ovulating, due to a failure of embryo implantation, as a consequence of decreased ovarian LH receptor (Lhcgr) expression and inadequate corpus luteum formation and progesterone production. To study further the presumed permissive role of PRL in the maintenance of gonadal responsiveness to LH/hCG stimulation, we crossed the hCGβ(+) and PRLR(-/-) mice. The double-mutant hCGβ(+)/PRLR(-/-) females remained sterile with an ovarian phenotype similar to PRLR(-/-) mice, indicating that LH action, Lhcgr expression, and consequent luteinization are not possible without simultaneous PRL signaling. The high frequency of pituitary prolactinomas in PRLR(-/-) mice was not affected by transgenic hCGβ expression. In contrast, none of the hCGβ(+)/PRLR(-/-) females showed either mammary gland lobuloalveolar development or tumors, and the increased mammary gland Wnt-5b expression, possibly responsible for the tumorigenesis in hCGβ(+) mice, was absent in double-mutant mice. Hence, high LH/hCG stimulation is unable to compensate for missing PRL signaling in the maintenance of luteal function. PRL thus appears to be a major permissive regulator of LH action in the ovary and of its secondary extragonadal effects.