Department of Obstetrics and Gynecology, McMaster University Medical Centre, Hamilton, Ontario, Canada, L8S 4J9, *Department of Medicine, Emory School of Medicine, Atlanta, Georgia 30303, U.S.A. and †Yerkes Regional Primate Center, Atlanta, Georgia 30322, U.S.A. (Received 26 April 1977) It is becoming evident that rodents and rhesus monkeys are not suitable models for the study of all aspects of human reproductive endocrinology (Hobson, Coulston, Faiman, Winter & Reyes, 1976). Apes, on the other hand, closely resemble man in many aspects of their endocrinology (Graham, 1976). As part of a continuing study on progesterone metabolism among the great apes, we have found that the single most abundant urinary metabolite of progesterone is 5β-pregnane-3α,20α-diol (pregnanediol) in the adult female chimpanzee (YoungLai, Graham & Collins, 1975) and is similar to that in the immature chimpanzee (Romanoff, Grace, Sugarman & Pincus, 1963). This is in contrast to the macaques where androsterone is the major metabolite