The effects of ovariectomy at day 12 of pregnancy on the proliferative activity of the uterine epithelium, metrial gland and uterine muscle in the rat were studied by autoradiography in animals killed one hour after a single injection of tritiated thymidine. The results were amplified by histological and histochemical studies. One day after operation epithelial proliferation was impaired in the ovarectomized animals. From 16 days onwards, however, proliferative activity was at a lower level in controls than in the ovariectomized animals. Three days after ovariectomy uptake of tritiated thymidine by metrial gland cells was less than in the controls. The metrial gland became smaller after ovariectomy and showed histological and histochemical changes from the normal pattern. The increased thickness of the uterine muscle in the ovariectomized animals from the 16 day stage onwards was not associated with any significant change in the labelling index of the muscle cells. The results are discussed in relation to the changes in endocrine environment which may occur after ovariectomy.