The major metabolites from incubat¡on of 14C-progesterone with 700 × g cell-free homogenates of cerebral cortex, hypothalamus and pituitary gland from 5 male ba-boons and 4 male rhesus monkeys were 5α-pregnane-3,20-dione and 2Oα-dihydroprogesterone. Both compounds were identified by their chromatographic properties, derivative formation, reverse isotope dilution methods and recrystallization to constant specific activities. The activities (pmoles h–1 mg–1) of the 20α-hydroxysteroid dehydrogenase, highest in the cerebral cortex of the subhuman primates, exceeded those of the 5α-reductase and did not change in males with sexual maturity. These data from subhuman primates are contrasted to those from rats in which the 5α-reductase activity exceeded approximately 2-fold that of the dehydrogenase in the 3 neural areas studied. In addition, 5β-pregnane-3,20-dione was isolated and identified from the pooled radioactive metabolites from incubation of the baboon neural tissues.