A critical investigation was made into the test described by Jayle et al. (1957) for investigating the endocrine activity of the testis. The conclusion was reached that the results of the administration of 3 × 10 000 IU chorionic gonadotrophin (HCG) (Test I by Jayle et al. 1957) can be more advantageously expressed by the increase of 17-KS during a period of 12 days, starting on the first day of HCG injection, than by the increase in 17-KS excretion on the third day of HCG administration (the criterion suggested by Jayle et al. 1957). In the form in which the test with HCG was carried out by us, however, it still cannot be regarded as an ideal test for the endocrine activity of the testis.