An investigation was carried out to define the ontogeny of normal mouse mammary gland responsiveness to the proliferative effects of estrogen (E) and/or progesterone (P). Since hormone receptors for both estrogen (ER) and progesterone (PgR) are present in both epithelial and stromal cells, we have investigated how the effects of E and P are related to the presence of receptor activity in the epithelium and stroma. Intact or ovariectomized mice, between 3 days and 10 weeks of age, were used to study the effects of E and/or P on DNA synthesis, as determined by DNA histoautoradiography; the cellular distribution of ER and PgR was investigated by steroid autoradiography. The results indicate that the mammary gland sequentially acquires the ability to respond to the stimulatory effects of E and/or P. In the early postnatal period (3-14 days) neither hormone was effective. Both epithelial and stromal cells first became responsive to E at 3-4 weeks of age. Estrogen receptors were first detected in stromal cells at 5 days of age and in epithelial cells at 2 weeks of age. Thus, the acquisition of estrogen responsiveness did not appear to be tightly coupled to the presence of ER in either epithelial or stromal cells. In contrast, responsiveness to P was acquired significantly later, at 7 weeks of age, and was closely linked to the presence of E-inducible PgR in epithelial cells. P caused a highly synergistic effect on epithelial cell DNA synthesis when combined with E, providing further support for the concept that the major proliferative effect of P is mediated via E-inducible PgR. PgR were also present in stromal cells, but the proliferative effect of P in that cell type was not correlated with the presence of PgR.