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The specific activity of epididymal 5alpha-reductase (pmol 5alpha-reduced products mg protein-1 h-1) decreased by 17, 44, 58 and 83% of the initial value and its total activity (nmol 5alpha-reduced products organ-1 h-1) decreased by 66, 85, 94 and 98% 2, 4, 8 and 14 days respectively, after castration. The loss of total activity always exceeded the decrease in organ weight and protein content. The decline in enzymic activity could be prevented by implantation of testosterone at the time of castration. Administration of testosterone propionate (200 microgram/day) for 12 days starting 1 month after castration was associated with the weights of the accessory organs returning to the control values and although the specific activity of 5alpha-reductase was almost completely restored by this treatment, the total activity of the nuclear fraction remained at 49% of the control value. Recombination experiments demonstrated that the effect of androgens is not mediated by a factor present in the soluble fraction and the concomitant administration of androgen and either cycloheximide or actinomycin D blocked the effect of androgen. These data suggest that androgens stimulate the synthesis of epididymal 5alpha-reductase.

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