Two groups of prepubertal unilateral cryptorchid boys (UCT) underwent stimulation tests with 1 μg (n=10) or 25 μg (n=8) of LHRH. Two groups of prepubertal bilateral cryptorchid boys (BCT) were similary studied (1 μg, n=6; 25 μg, n=8). The number of spermatogonia was determined in both sides in each group after testicular biopsy. UCT groups exhibited FSH and LH responses to LHRH similar to those of prepubertal control groups. Mean numbers of spermatogonia were low on the cryptorchid side (12 ± 6 to 25 ± 12) and normal on the normal side (>50). Both groups of BCT had very low counts of spermatogonia on both sides (means of 7 ± 4 to 14 ± 8). Their FSH responses to LHRH at both dosages were significantly higher than those of the normal prepubertal controls. LH responses were identical. These data suggest that the negative feedback mechanism between FSH secretion and the spermatogonia observed in the adult male is also operative in the prepubertal boy.