1. Castration of male albino rats, aged 30 weeks at death, caused reduction in volume of the sebaceous glands by diminishing the size of the cells and their rate of proliferation. Cell life or 'turnover time' appeared to be increased.

2. Implantation of testosterone in castrated rats increased the volume of the glands by increasing the size of the cells and their rate of proliferation; at the same time cell life was reduced.

3. Testosterone had no effect on the size of the sebaceous glands in hypophysectomized castrated rats.

4. Nevertheless, in hypophysectomized castrated animals treated with testosterone there appeared to be larger sebaceous cells than in similar untreated animals, and a higher rate of cell proliferation in the sebaceous glands than in untreated castrated rats. The explanation of this anomaly may be that in all hypophysectomized castrated animals, and particularly in those treated with testosterone, cell life was very short. These results suggest that the pituitary does not exert a clear-cut, 'all or none' effect but rather facilitates the full action of testosterone on the sebaceous gland.

5. Testosterone increased the rate of cell proliferation in the epidermis of castrated rats, but this did not affect the thickness of the Malpighian layer.

6. Hypophysectomy of castrated rats significantly increased the thickness of the Malpighian layer, and this appeared to be due to an increased rate of cell proliferation over that of castrated rats rather than to an increase in the length of cell life. Testosterone did not reduce epidermal thickness in these animals.