Endometrial histology in the chimpanzee (Pan troglodytes) was found to be essentially similar to that of the human throughout the menstrual cycle. Proliferative features were associated with development of the sexual swelling, and the first appearance of secretory changes was closely associated with detumescence of the swelling. Administration of a synthetic estrogen to ovariectomized chimpanzees resulted in the development of sexual swelling and proliferative endometrium. Administration of progesterone to estrogen-primed animals resulted in detumescence of the swelling and development of secretory endometrium.