The number of cells that synthesize DNA in the uterine luminal epithelium of ovariectomized mice and in the uterine connective tissue stroma of progesterone-treated mice reaches a maximum approximately 15 h after oestrogen is given (Martin, Finn & Trinder, 1973a, b). Martin & Finn (1968) maintained that the time-course of the mitotic response was similar in both tissues, but DNA synthesis has been shown to be of 3 h shorter duration in the oestrogen-stimulated epithelium than in the stroma (Das, 1972), and on this basis the time-courses of the mitotic responses might be expected to differ. Albino mice were ovariectomized under tribromoethanol (Avertin) anaesthesia. Five days later they were primed with 100 ng oestradiol s.c. for 3 days. Starting 2 days after priming, groups of five mice received daily injections of 1 mg progesterone s.c. for 4 days, and 50 ng oestradiol were given with the last injection. Other