Testosterone concentration has been measured in spermatic and peripheral venous plasma obtained during surgery from a total of 25 prepubertal boys affected either by inguinal hernia (Group I; N = 6; age range 2-8 years) or unilateral undescended testis (Group II; N = 19; age range 5-11 years). Median spermatic venous testosterone level was 58.7 ng/dl) (range 14.0--120.8 ng/dl) in Group I and 43.2 ng/dl (range 12..2-267.5 ng/dl) in Group II; median peripheral testosterone level was 4.9 ng/dl (range 2.3-15.4 ng/dl) and 5.6 ng/dl (range 1.1-89.3 ng/dl) in Group I and II, respectively. The difference between the spermatic and peripheral level was statistically significant in both groups (P less than 0.01 in Group I and P less than 0.001 in Group II). These results indicate that the prepubertal human testis secretes testosterone, even if in a very low amount. It is also suggested that this secretion can be responsible for LH inhibition in prepubertal boys.