The uptake and retention of 1, 2-<sup>3</sup>H-testosterone in accessory sex glands, muscle and liver of streptozotocin diabetic castrated male rats, insulin-treated diabetic castrated rats and non-diabetic castrated control rats were studied at various time intervals after an intravenous injection. Diabetes reduced the retention of <sup>3</sup>H-testosterone in the prostate, the preputial gland and the epididymis. Exogenous insulin slightly increased the retention of <sup>3</sup>H-testosterone in these tissues of diabetic rats. No significant differences in the radioactivity in the rectus abdominis muscle, the coagulating glands and the seminal vesicles were found between the various experimental groups. Ventral prostate homogenates obtained from diabetic and control rats were incubated with <sup>3</sup>H-testosterone in vitro. The steroids were extracted and thin-layer chromatographs were scanned for radioactivity. In prostatic homogenates taken from diabetic rats, testosterone transformation to dihydrotestosterone was reduced. The results indicate that the impaired function and androgen retention of the accessory sex glands of diabetic male rats is at least partly due to the reduced formation of dihydrotestosterone from testosterone.