Plasma testosterone has been studied in 31 full-term male infants born with bilaterally undescended tests (14) or unilaterally undescended testis (17). From 10 to 89 days after birth, the post-natal testosterone rise was significantly lower in the 18 infants who remained cryptorchid at 4 mo. than in the 13 who underwent spontaneous testicular descensus and in the normal controls. Blunted post-natal Leydig cell secretion in cryptorchids may relate to a primary LH [lutropin] defect and could contribute to the impairment of both testicular descensus and maturation.