The influence of estradiol benzoate and of progesterone on the distribution of phospholipid in the endometrium of the castrate rat has been studied in tissue sections prepared by the controlled chromation method for phospholipids and the direct silver method for the Golgi apparatus. Estrogen decreases the basal phospholipid and causes a rapid growth of the Golgi apparatus, effects which are not paralleled by progesterone. The response of both the general phospholipid and the Golgi apparatus to graded doses of estradiol benzoate has been investigated.