1. The role of testosterone (T) in growth was evaluated in 11 prepubertal hypopituitary males during two 15-day periods separated by a 4-week interval, i.e., before (PRE-T period) and during T ester treatment (50 mg every 5 days, 3 im doses-T period). 2. T increased growth hormone (GH) secretion, assessed by 4-h rhythm (mean +/- SEM = 1.90 +/- 0.27 vs 1.77 +/- 0.21 ng/ml; P < 0.05) and after a GHRH stimulus (3.42 +/- 0.54 vs 3.08 +/- 0.43 ng/ml; P < 0.05) as compared to the PRE-T period. 3. T also increased basal somatomedin-C (SM-C) levels (0.20 +/- 0.03 vs 0.15 +/- 0.02 U/ml; P < 0.001) and SM-C generation. After GH was administered in 4 im doses (0.01, 0.02, 0.05 and 0.1 U/kg), SM-C levels were 0.31 +/- 0.08 vs 0.24 +/- 0.07 U/ml, P < 0.001. T did not change incremental (absolute minus basal) SM-C levels (0.15 +/- 0.08 vs 0.12 +/- 0.07 U/ml; P > 0.05). 4. The results suggest that T increased plasma SM-C levels by stimulating residual GH secretion in hypopituitary males.