Mitochondria, dense bodies, lysosomes, lipid, and rough endoplasmic reticulum (RER) were quantitated in the endometrial stromal cells of ewes after either ovariectomy alone or ovariectomy and treatment with estradiol or progesterone. The most striking change in the endometrial stromal cell was seen in the RER. After ovariectomy, the RER was scant and poorly developed. In ewes treated with estradiol, the RER was abundant and extensively dilated. In progesterone-treated ewes, the RER was less well-developed than in estradiol-treated ewes but somewhat more abundant than in ovariectomized control ewes. The increased RER in estradiol-treated ewes probably reflects increased protein synthesis by the endometrial stromal cells.