LH-RH test and HCG stimulation test were performed in 154 cryptorchid boys aged 1 month to 15 years (64 unilateral and 90 bilateral). Basal plasma LH levels and LH response to LH-RH were significantly lower from infancy to early puberty in cryptorchids compared with controls. Basal FSH levels and FSH response to LH-RH were normal. The post-HCG rise of plasma testosterone was reduced until mid-puberty. A significant positive correlation was found between post-HCG testosterone levels and pre- and post-LH-RH levels of LH. This correlation suggests that testicular maldescent and the decreased ability of Leydig cells to respond to a short course of HCG may result from an early defect or a delay of pituitary LH secretion.