Testicular testosterone (T) production was examined in thirty boys with undescended testes (UT) following the administration of 4500 U gonadotropic hormone. Twenty boys had bilateral UT and ten had UT plus hypospadias. As for possible causes of reduced Leydig cell activity it was investigated whether the testis was (1) hypoplastic; (2) abnormally fused with the epididymis; (3) located in the abdomen; (4) or UT was associated with hypospadias. Average T values were significantly lower when the testicle was hypoplastic or its fusion with the epididymis was imperfect; but remained largely undiminished when the testicle was located in the abdomen or when UT was combined with hypospadias. The occurrence of both pathologic and physiologic T reactions in each of the four groups suggests that the population of UT children is heterogeneous, probably due to differences in aetiology and in intrauterine hormonal processes. In the case of UT and hypoplasia the time and method of operation (orchidopexy) must be selected with utmost care, bearing in mind that an originally small testicle with impaired T secretion may become physiologic by the time of puberty.