The histologic characteristics of baboon endometrium during the menstrual cycle and pregnancy were studied and compared with those of human endometrium. Eight phases of endometrial change during the menstrual cycle are described. The samples were dated on the basis of sex skin changes. The basic histology of the baboon endometrium is similar to that of human endometrium, but some differences were observed. The growth of baboon endometrium is more sluggish, secretion is less intense, and many of the changes, especially in the stroma, are localized, less diffuse than they are in human endometrium. During pregnancy, decidual transformation in the baboon is much less intense than that in the human. No differences were observed between endometrium in association with preimplantation embryos and that from nonpregnant animals at comparable times following ovulation.