The effect of age on the metabolism of progesterone was studied in the rat brain. Metabolic activity was more active in minced tissues than total homogenates. The activity of progesterone 5α-reductase(s) was increased during postnatal periods(between 5 and 14 days after birth) and thereafter steadily decreased up to the one-fourth level of the fetus. When 5α-dihydroprogesterone was incubated with brain tissues of various ages, the change in the activity of 3α-hydroxysteroid oxidoreductase(3α-HSOR) was similar to that of 5α-reductase(s). These results suggest that the reduced formation of total 5α-reduced metabolites was due to the decreased activities of 5α-reductase(s) and 3α-HSOR. However the level of 3β-HSOR remained constant regardless of the age.