The androgen 5α-dihydrotestosterone (DHT) has been isolated and identified in testicular tissue (rats, dogs) and in testicular venous blood of the canine testis. Conversion of 3H-testosterone (T) to 3H-DHT in vitro by testicular tissue from rats treated with follicle-stimulating hormone in vivo tended to increase. No such effect of human chorionic gonadotropin treatment could be observed. The Leydig cells of testes from mature rats contribute little to the formation in vitro of 3H-DHT from 3H-T. Both these androgens can be found in spermatic lymph and in epididymal tissue of the dog. In an isolated prostate preparation where the gland is infused via the prostatic arteries with the animal’s arterial blood at a constant rate, DHT can be found in effluent blood. The dog prostate both produces and secretes DHT. The purpose of DHT production and secretion by male reproductive organs is discussed.