The distribution of the progesterone receptor (PR) was investigated immunocytochemically in female reproductive tracts of rats during the estrous cycle and early pregnancy through use of an anti-PR monoclonal antibody. PR was localized predominantly in the nuclei of epithelial, stromal, and muscle cells in the uterus and vagina during the estrous cycle. In the uterus, the nuclei of epithelial cells were stained intensively at diestrus, while the PR staining of the stromal cells was more intense at proestrus than at any other stage of the cycle. PR expression during the cycle in muscle cells of the myometrium was similar to that in the endometrial stromal cells. In the vagina, however, PR expression during the cycle was approximately the same among epithelial, stromal, and muscle cells, the nuclei of which were stained deeply at proestrus. Ovariectomy at various stages of the cycle altered the PR expression appearing in the uterus and vagina during the cycle. In ovariectomized rats, estrogen increased the PR immunoreaction of various types of cells examined in the uterus and vagina except for the uterine epithelial cells. The reaction of these uterine epithelial cells was decreased by estrogen but was increased by progesterone given after estrogen; however, progesterone given alone reduced the reaction. In the epithelial and stromal cells of the uterus, intensity of the staining was increased after mating, reaching maximum on Day 3 of pregnancy, and then decreased on Day 4 (day of implantation), while in epithelial and stromal cells of the vagina the staining remained weak during early pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)