In order to investigate the in vitro steroidogenic capacity of glands at different ages, minces of testicular tissue were incubated with labelled precursors. The studies were conducted in triplicate at 35°C. For equal quantities of incubated tissue the non-metabolized amount of [ 3 H]pregnenolone and [ 14 C]progesterone, utilized as precursors, was different in post-pubertal and senescent animals: ± 3 vs ± % ( P < ) for pregnenolone, and ± vs ± % ( P < ) for progesterone, respectively. Testosterone production was 12 ± 2% in adult and ± % in senescent animals ( P < ). The testosterone/ androstenedione ratio was not significently different in post-pubertal and senescent animals: ± vs ± , but consistently higher than found in immature animals: ± . The lesser potential capacity of the aging tissue to synthesize testosterone could be explained by a decline in the glands capacity to metabolize the hormonal precursors.