The number of mitoses per unit area, or length of tissue, has been estimated in the epidermis of the ear, and in the epithelial linings of the oesophagus, duodenum and urethra. Specimens were taken from female rats in pro-oestrus, oestrus and the 1st and 2nd days of dioestrus, and from spayed rats which had received either no treatment or up to five injections of 25 μg of oestrone. The results of these experiments do not confirm claims, made for various mouse tissues, that the rate of mitosis varies periodically in phase with the oestrous cycle, or that it is increased by oestrogen.