The action of testosterone (T), dihydrotestosterone (DHT) and 5.alpha.-androstane 3.alpha., 17.beta. diol (Diol) on the seminiferous epithelium of immature estrogen-treated rats was studied. Quantitative histological evaluation was performed and testicular proteins and androgens contents were determined. Estradiol benzoate (E2B) treatment resulted in a severe testicular atrophy with a reduction of 50% in the testicular weight and protein content. In the estrogen treated rats the levels of androgens in the testicular tissue were undetectable. Histologically, the germinal epithelium showed a marked decrease of cell types, particularly the meiotic spermatocytes and young spermatids. In E2B+T, E2B+DHT and E2B+Diol-treated animals testicular weight decreased only 10-20%, with a normal concentration of testicular proteins. In E2B+T-treated animals the 3 androgens were found in the testis, and in both E2B+DHT and E2B+Diol-treated rats, high levels of these androgens were found, but testosterone was not detectable. In every case the levels of Diol were higher than those of DHT. A diminution of spermatogonia, similar to that found in E2B-treated animals was noted in all androgen-treated rats. Meiotic spermatocytes showed a significant increase in relation to E2B-treated rats, even though a decreased was observed when compared with the normal controls. Spermatid diminution was only partially prevented in E2B+DHT and E2B+Diol-treated groups, but not in E2B+T treated rats. T, DHT and Diol exert a meiotic stimulatory effect. In the case of DHT and Diol-treated animals, absence of testicular indicates that both 5.alpha. reduced metabolites are biologically active in the prevention of testicular atrophy induced by E2B at the time of initiation of spermatogenesis.