Testis regression was induced in male Syrian hamsters by chronic exposure to diethylstilbestrol (DES), and estradiol-17 beta agonist. Experimental groups (n = 4-5) were killed at increasing time intervals over a period of 6 mo after initiation of treatment. Apoptosis in testes was demonstrated by in situ analysis of DNA fragmentation. Cell proliferation was monitored by immunostaining nuclei of S-phase cells after pulse labeling with 5-bromo-2'-deoxyuridine. Levels of FSH and testosterone, measured by RIA fell rapidly in DES-treated hamsters. In parallel, testis weight and seminiferous tubule area underwent an 80% decrease during the first 2 wk of DES administration. The composition of seminiferous epithelium was also drastically affected by DES, since it became progressively confined to Sertoli cells, spermatogonia, and spermatocytes. Testis regression was associated with an important increase of apoptosis, which started 3 days after the beginning of DES administration. Apoptosis was still 10- to 50-fold higher than in control testes by the end of treatment; it affected primarily spermatocytes and, to a much lesser extent, spermatogonia. Cell proliferation was not inhibited by chronic DES administration. In conclusion, these data indicate that apoptosis can by itself account for estrogen-induced testis regression.