The age dependence of Leydig cell function was investigated in rats from prepuberty (15 days) to senescence (39 months). Serum LH, serum and testicular testosterone were measured by radioimmunoassay. The binding capacity and affinity of LH/hCG receptors were determined by a radioligand receptor assay (hCG/Leydig cells) using 125I-hCG labelled by the lactoperoxidase method. Separation of bound and free 125I and simultaneous concentration of 125I-hCG was achieved by vacuum ultrafiltration. The biochemical integrity of 125I-hCG tracer was ascertained by various chromatographic procedures. The highest hCG-binding and highest serum LH levels were found during puberty. Serum and testicular testosterone concentrations, however, were maximal in early adulthood. From this period onwards to late senescence hCG-binding changed only slightly, while serum LH and testosterone levels decreased significantly towards late senescence. The study shows that, although hCG binding to the Leydig cell changes characteristically during development, it is minimally affected by aging and cannot therefore be responsible for the reduced androgen biosynthesis in senescence.