SUMMARY: The duration of the phase of DNA synthesis (S-phase) and turnover times were measured by autoradiography in the vaginal and endo-uterine epithelium of spayed mice at different times after a single oestradiol injection. With three doses of oestradiol (0·3, 1·2 and 10·0 μg), changes of turnover time followed similar patterns in uterine epithelium, but maximal activation and subsequent levelling off of proliferation rates were reached sooner with higher doses. In the vagina, no significant dose-dependence was apparent with the same doses of oestradiol; maximal activation of cell proliferation was observed in every case 12 h after hormone injection. Transient shortening of S-phase was observed in both tissues under oestrogen stimulation. The morphological changes in uterine epithelium appeared to be independent of mitotic stimulation. For vaginal epithelium, the results may be interpreted to indicate that oestrogen-induced keratinization starts by a differentiating action on pre-existing G1 cells.