Abstract In various species (rat, mouse, ox, swine, rabbit, chicken, golden hamster, chinchilla, guinea pig, dog, man) NADH- and NADPH-5α-reductase activities were determined in liver microsomes. The specific activities were highest in rat, hen and golden hamster (1–35 nmol/min/mg). The other species contained a specific activity of less than 1 nmol/min/mg. In most species higher NADH- and NADPH-5α-reductase activity was found in the female animal; the golden hamster is the exception where the sex-difference in 5α-reductase activity is reversed with higher specific activity in the male animal. The pH-optima of NADH- and NADPH-5α-reductase were determined in all species.