In normal mouse mammary gland, the mitogenic action of progesterone (P) is mediated by two P receptor (PR) isoforms, PRA and PRB. PRA is predominantly expressed in the adult virgin, and PRB is predominantly expressed during pregnancy. To investigate hormonal regulation of PR isoform expression and isoform-specific functions in vivo, adult ovariectomized BALB/c mice were treated for 3, 5, or 10 d with estrogen (E), P, or estrogen plus progesterone (E+P). Using an immunohistochemical approach with isoform-specific antibodies, we investigated hormonal regulation of PRA and PRB and their functional roles in proliferation and morphogenesis. Significant E-induced proliferation was only observed after 5 d at the distal tips of ducts; there was no sidebranching or alveologenesis. P induced proliferation that resulted in sidebranching and alveologenesis, but E+P treatment produced more proliferation sooner and more extensive sidebranching and alveologenesis. PRA levels were increased by E and decreased by P. Increased PRB levels were induced by treatment with P or E+P and coincided with the formation of alveoli. PRA was the predominant PR isoform expressed during sidebranching, and colocalization of PRA with 5-bromo-2′-deoxyuridine revealed that proliferation of PRA-positive and -negative cells was responsible for P-induced sidebranching. PRB was the predominant PR isoform expressed during alveologenesis, and colocalization of PRB with 5-bromo-2′-deoxyuridine showed that both PRB-positive and -negative cells proliferated during alveolar expansion. These results demonstrate different hormonal regulation of PRA and PRB levels in vivo and suggest that P can induce proliferation through either PRA or PRB via direct and paracrine mechanisms.