Testicular biopsies of 6 adult patients operated for chromophobe adenoma have been studied before and after treatment with HCG, HMG, and HMG with HCG. Control biopsies were obtained at different intervals of time in each case, ranging from 6 months to 10 yr after operation. Based on the extent of the regression of germinal epithelium, tubular wall and interstitial cells, the patients were classified in 2 groups, each one comprising 3 patients. Group I showed germinal cells up to spermatocyte level accompanied by moderate peritubular hyalinization and partial atrophy of Leydig cells. Group II only showed spermatogonial and Sertoli cells, accompanied by intense peritubular hyalinization and almost complete atrophy of Leydig cells. In order to estimate the variations in the relative number of germinal and Leydig cell populations before and after treatment, a cell count was applied to histological sections. The 3 patients of the 2 groups received treatment during 3 months with one of the following hormones: HCG, HMG, or a combination of HMG plus HCG. Results obtained were: a) HCG seems to have a stimulating effect on the spermatogonial phase, thus giving rise to primary spermatocytes. This action is accompanied by the simultaneous stimulation of Sertoli and Leydig cells and reduction of peritubular hyalinization. b) HMG stimulated all germinal cell phases, particularly spermatogonial, spermatocytes and less evidently the spermiogenic phase; the effect on Leydig cells was weak and peritubular hyalinization tended to diminish, c) Addition of HCG to HMG resulted in the complete recovery of spermatogenesis, together with full development of Leydig cells, repair of Sertoli cells and disappearance of hyalinization; this response was better seen in those cases having pre-existent spermatocytes than in those showing only spermatogonial cells. d) As, in these cases, the stimulating effect on all germinal cell phases does not reach the level of the cell population seen in normal subjects, only a qualitative progression of spermatogenesis was achieved. e) Based on the different ratios of FSH and LH activity contained in HMG and HCG, it is speculated that both hormones may act simultaneously on all germinal cell phases and that a high LH/FSH ratio may be necessary for the completion of human spermatogenesis.