The effects of in vivo administration of corticotrophin (ACTH) and human chorionic gonadotrophin (HCG) on rat testes were studied. HCG, administered for 14 days to young rats, led to testicular interstitial cell hypertrophy and hyperplasia, accompanied by stimulation of in vitro testicular androgen production, as manifested by increased 17\g=a\-hydroxylase and 17\g=b\-desmolaseenzyme activities, with no clear-cut effect on 20\ g=a\ \ x=r eq-\ hydroxysteroid dehydrogenase activity. The findings in ACTH-treated animals were similar to those in the saline-treated controls. The lack of influence of ACTH on the testicular enzymes involved in steroid biosynthesis is consonant with the concept that ACTH-dependent bilateral testicular tumours occurring in boys with congenital adrenocortical hyperplasia arise from [00BB]adrenocorticalrest cells[00AB]rather than from testicular interstitial cells.Bilateral steroid-secreting testicular tumours occurring in a boy with the saltlosing form of congenital adrenocortical hyperplasia (Schoen et al. 1961) aroused our interest in the mechanism of formation of such tumours. Results of in vitro studies of steroid biosynthesis in these tumours favoured the theory that the tissue of origin was abnormal »adrenocortical rest cells« in the testes rather than testicular interstitial cells (Domínguez 1961).