The rates at which cell renewal occurs were determined by the colchicine technique in adult rats. The percentages of cells newly formed in the epithelia during one estrous cycle were ascertained, and their turnover times calculated; these data are as follows: Cells formed during Turnover time one estrous cycle in days Vagina 114% 3.9 Cervix uteri 81% 5.5 Uterine surface epithelium 75% 5.9 Endometrial glands 43% 10.4 Uterine tube 11% 41 Ovarian germinal epithelium 14% 33 Data are provided also on the mitotic rates of ovarian follicles and corpora lutea. Prominent fluctuations of mitotic rate were observed during the estrous cycle in some epithelia, as well as diurnal variations throughout the day. Cell renewal occurred by mitosis particularly of basal cells in stratified squamous epithelia (vagina, cervix), and of cells in basal glandular portions (endometrial glands.). This cellular new formation compensated for cells simultaneously lost from the epithelia and glands.by desquamation and disintegration.