Radioautography of the uterus and vagina after [3H]progesterone injection into guinea pigs at various periods of the estrous cycle.

Radioautograms were made of uterine horns, cervix and vagina from guinea pigs injected with[3H]progesterone at various periods of the estrous cycle. The concentration of silver grains peaked at proestrus and estrus and then fell during metestrus, attaining a minimum at diestrus. These variations were not due to masking of binding sites by endogenous hormone since ovariectomy of guinea pigs at diestrus did not change this pattern. During proestrus and estrus, radioactivity was found in all parts of the endometrium and myometrium of the uterine horns. However, the muscle fibers and the cells of the stroma contained more silver grains than did the luminal and glandular epithelium. In the vagina, radioactivity was concentrated mostly in the nuclei of the basal layer cells of the stratified epithelium. During metestrus and diestrus, less radioactivity was present in these regions. These differences in labelling were observed simultaneously in all the uterine and vaginal cell types, suggesting that similar hormonal mechansims may control receptor variations in these different target cells.