A study was made of the changes in morphology and in nuclear incorporation of tritiated thymidine in the rat uterus and placenta after ovariectomy at day 10 of gestation. There was some individual variation between different animals in the results of the ovariectomy, but in general the effects on both maternal and fetal tissues were more severe than those reported to follow ovariectomy at later stages of pregnancy. Even in the animals where there was extensive placental survival 2 or 3 days after the ovariectomy, normal differentiation of the placental labyrinth did not occur and the fetuses failed to survive. Ovariectomy had a pronounced effect on the proliferative activity of uterine epithelial cells. The high labelling index in the control animals up to day 12 was markedly reduced in the ovariectomized animals; after day 13, however, the labelling index of the controls was reduced to a level lower than that of the ovariectomized animals. Nuclear labelling occurred in the glandular epithelium from two days after ovariectomy, but was never present in the controls. There was a marked reduction in the percentage of labelled nuclei in the uterine muscle and in the metrial gland after ovariectomy. In the metrial gland this was associated with a reduction in the number of typical granulated cells and with the appearance of numerous small round cells. It is suggested that the latter represented metrial gland cell precursors which had undergone impairment of their normal differentiation process.