Administration of testosterone propionate to immature male rats caused a 50% increase in the specific activity of glucose 6-phosphate dehydrogenase, the rate-limiting enzyme of the pentose phosphate pathway, in the levator ani muscle. This effect appears to be mediated by conversion of testosterone to estradiol because of the following results: 1) the effect was not mimicked by the nonaromatizable androgens fluoxymesterone or 5 alpha-dihydrotestosterone; 2) it was mimicked by 17 beta-estradiol and diethylstilbestrol; 3) it was blocked by an estrogen antagonist but not by an androgen antagonist; 4) the inactive steroid 17 alpha-estradiol was without effect on glucose 6-phosphate dehydrogenase; and 5) the effect of testosterone was blocked by an inhibitor of androgen aromatase. These results demonstrate a direct effect of estrogen on striated muscle.