AbstractThe purpose of this study was to ascertain the effects of different amounts of progesterone on estrogen‐induced DNA synthesis in the vaginal and lower cervical epithelia of the rat. Ovariectomized rats were injected subcutaneously with 1 μg of estradiol‐17β dipropionate or with estradiol and 1, 5, 10 or 15 mg of progesterone for three days; control rats received oil. Tritiated thymidine was injected one hour prior to necropsy; the tissues were processed for autoradiographic study and the percentages of labeled nuclei in the basal layer from the lower half of both the vagina and cervix were determined. In the group of rats given estradiol and 1 mg of progesterone the thymidine uptake in the vagina was significantly increased over that of rats given only estrogen. However, in the rats treated with estradiol and 5, 10 or 15 mg of progesterone, the extent of DNA replication in the basal epithelial nuclei was similar to the estrogen group. These responses were compared with those of the basal epithelium from the lower half of the cervix. None of the estrogen‐progesterone treatments increased the thymidine index in the cervical epithelium above that of estrogen alone; additionally, 5, 10 or 15 mg of progesterone given with estrogen suppressed DNA replication. These data indicate that stratified epithelia of these two portions of the reproductive tract respond differently to exogenous ovarian h