Physiological amounts of testosterone- 4–14C were infused into the common arterial supply of the prostate gland and urinary bladder of 5 adult dogs. Only the labeled hormone was found in blood from the inferior vena cava 2 min after the beginning of the infusion until the dog was killed 9 min later. Recoveries from the prostate and urinary bladder accounted for 1–5% and 0.9–3%, respectively, of the substrate radioactivity. Metabolism of the infused testosterone- 4–14C in the canine prostate was extensive and yielded almost exclusively 5α-reduced products. There was a several-fold predominance of 17β-hydroxy metabolites over the 17-ketone analogs and of 3α-hydroxy metabolites over the 3β-epimers. Half the prostate radioactivity was associated with 2 major transformation products, 5α-dihydro testosterone (17β-hydroxy-5α-androstan- 3-one) and 5α-androstane-3α,17α-diol. In contrast, metabolism of testosterone-4-14C in the urinary bladder was much more limited and oxidative, with 4-androstene-3,17-dione as the major and 5α-androstane-3,17-dione as the minor product.The different pathways of male hormone metabolism in tissues which have a common embryonic origin yet are unlike in their response to androgen and the identification of 2 major prostate metabolites with distinct biological modes of action suggest that testosterone 5 areductase and 3α9-hydroxy-C19-steroid oxidoreductase are systems which regulate androgenic activity at the target site. (Endocrinology87: 394, 1970)