SUMMARY 1. In the dorsal skin of female albino rats, the sebaceous glands reached their maximum size, and the stratum germinativum its greatest thickness during prooestrus. At oestrus, and on the day following it, both these tissues were significantly reduced in size. 2. During the oestrous cycle, there appeared to be no significant changes in the incidence of mitoses in either tissue, as measured 5 hr after injection of colchicine. 3. An analysis of correlation showed that in neither tissue was size correlated with incidence of mitoses. Sebaceous gland size was very significantly correlated with epidermal thickness, and the two incidences of mitoses were themselves correlated. 4. In immature female rats, which had been implanted with oestradiol benzoate 3–4 weeks before death, both sebaceous gland size and epidermal thickness were greatly reduced, although the incidences of mitoses were significantly raised. 5. These results are interpreted as indicating that oestrogens affect both holocrine secretion of the sebaceous glands and epidermal keratinization, and that these actions are independent of any possible influence on the incidence of mitoses in these tissues.