Whole homogenates of testes from rats of different sizes have been incubated with progesterone-4-14 C and labeled products have been isolated and assayed for radioactivity. A marked change in the pattern of metabolites takes place when the rats are 200-300 g in weight (7-9 weeks). With testes from 150-200 g animals testosterone is formed in appreciable yields but to a great extent is further metabolized, principally to 5a-androstane-3a,17/?-diol. The ring A reductase activity responsible for this transformation appears to decrease during an early phase of rapid sexual maturation. It is suggested that this change in testicular enzymatic activities may play a role, and that 5«-androstane-3a,17/3-diol may serve some special function in the process of sexual maturation. {Endocrinology 78:1041,1966)

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