Effects of diethylstilbestrol (DES) treatment (30 daily i.m. injections) in reproductive function in the male rat were investigated. These functions were evaluated with sperm counts, motility, fertility, morphology, serum testosterone, and a number of biochemical tests in rat epididymis and vasa deferentia. DES treatment resulted in a 15% loss of body weight and a 60 to 90% loss of weight of testes, epididymis, and vas deferens. Sperm parameters and fertility were reduced to nil by DES treatment. Likewise, the majority of the biochemical measures were severely depressed by the DES treatment. In general, the measured parameters were less affected or recovered with either testosterone cotreatment or 60 days of recovery, showing the role of androgen in maintaining functional integrity and metabolism. However, sperm parameters and fertility did not recover with testosterone cotreatment, which differs from withdrawal of treatment.