To obtain more information about testicular inhibin secretion in the prepubertal and pubertal human male, we measured the concentrations of immunoreactive inhibin with a heterologous radioimmunoassay in the spermatic and peripheral venous plasma of 5 prepubertal boys with unilateral undescended testis (Group I, PI), 3 prepubertal boys with inguinal hernia (Group II, P1), and 12 pubertal boys with left idiopathic varicocele. The latter subjects were divided, according to the degree of their pubertal development, in early pubertal (Group III, N = 5, P2) and mid-pubertal groups (Group IV, N=7, P3–4). In Group I, the mean (±sd) spermatic venous concentrations of inhibin (289.4±120.4 ml eq/l) were significantly higher than the corresponding mean peripheral venous concentrations (162.6±47.2; p<0.02) suggesting active testicular secretion of inhibin. In Group II, the spermaticperipheral inhibin gradient was not significant. In pubertal boys with idiopathic varicocele, the mean concentrations of spermatic inhibin were 1076.6±532.0 and 1023.4±274.5 in Groups III and IV, respectively. These levels were about five times higher than the corresponding peripheral concentrations (204.8±41.9 and 238.9±38.9; p<0.005 and p<0.001, respectively). When the data of all the boys were considered together the spermatic venous concentrations of inhibin were significantly correlated with those of peripheral venous FSH (r=0.4749, p<0.05). The spermatic venous inhibin concentrations and the spermatic/peripheral inhibin ratios were significantly correlated also with peripheral LH (r=0.5700 and 0.4766, p<0.01 and <0.05 respectively), spermatic testosterone (r = 0.6237 and 0.5893; p<0.05, respectively), and spermatic/peripheral testosterone ratios (r=0.7529 and 0.7313; p<0.01, respectively). Our results suggest: (a) the undescended testis of prepubertal boys secretes small but significant amounts of inhibin; (b) in pubertal boys with idiopathic varicocele the inhibin secretion of the testis of the affected side is more evident; (c) a contribution of Leydig cells to the testicular secretion of inhibin is likely.