Ovarian steroids have important inter-related roles in many systems and processes required for mammalian reproduction. The female reproductive tract, ovaries, and mammary glands are all targets for both estrogen and progesterone. In addition, the actions of these hormones are intertwined in that, for example, progesterone attenuates the proliferative effect of estrogen in the uterus, whereas estrogen also induces the progesterone receptor (PR) mRNA and protein, thus enhancing progesterone actions. The generation of mice that lacks the progesterone receptor (PRKO) or the estrogen receptorα (αERKO) has provided numerous insights into the interacting roles of these hormones. The mammary glands of the PRKO mice develop with full epithelial ducts that lack side branching and lobular alveolar structures, whereas the αERKO mice develop only an epithelial rudiment. This indicates that estrogen is important for ductal morphogenesis, whereas progesterone is required for ductal branching and alveolar development. Both the αERKO and PRKO mice are also anovulatory, but exhibit different causal pathologies. The αERKO ovary seems to possess follicles up to the preantral stage and shows a polycystic phenotype as a result of chronic hyperstimulation by LH. The PRKO follicles seem to develop to an ovulatory stage, but are unable to rupture, indicating a role for progesterone in ovulation. The uteri of these two strains seem to develop normally; however, the function and hormone responses are abnormal in each. Because estrogen is known to induce PRs in the uterus, the progesterone responsiveness of the αERKO uterus was characterized. PR mRNA was detected but was not up-regulated by estrogen in the αERKO tissue. PRs are present in the αERKO tissue at 60% of the level in wild-type tissue and show a similar amount of A and B isoforms when measured by R5020 binding and detected by Western blotting. The PRs were able to mediate induction of two progesterone-responsive uterine genes: calcitonin and amphiregulin. The αERKO uterine tissue was also able to undergo a decidual reaction in response to hormonal and intraluminal treatments to mimic implantation; however, unlike normal wild-type uteri, this response was estrogen independent in the αERKO uterine tissue.