Whole testicular tissue, isolated seminiferous tubules and interstitial tissue of developing rats were incubated with testosterone–14C. After incubation, several steroid metabolites were isolated by paper and thin—layer chromatography and derivative formation. They usually accounted for more than 90% of the radioactivity added as testosterone–14C. Losses were corrected with tritiated tracers. Identification of products was confirmed by recrystallization. 5α–Androstandiol– 14C was the main metabolite isolated from the tubules, dihydrotestosterone–14C and androsterone– 14C were other minor products. After 4 hr of incubation, percentage conversion into 5α–androstandiol– 14C at different ages was as follows: 1 day, 3.2; 10 days, 0.8; 11 days, 0.3; 12 days, 3.2; 13 days, 2.8; 14 days, 15.7; 16 days, 16.9; 18 days, 34.4; 20 days, 61.4; 45 days, 62.5 and 365 days, 32.6. In the interstitial tissue of 20–day—old rats, the percentage conversion into 5α–androstandiol– 14C was much smaller, 12.4, than in the corresponding tubules; most of the testosterone–14C remained unchanged at 84.3. Microscopic examination of the testes showed that formation of 5α–androstandiol– 14C correlated very closely with development of meiosis. In sexually mature rats, 5α–androstandiol–14C was also the main metabolite of testosterone–14C but the percentage conversion was lower than in 20– or 45–day—old animals. (Endocrinology91: 537, 1972)