The antiandrogens flutamide and casodex have been administered subcutaneously (vehicle, 1, 5 or 10 mg per day) to prepubertal male rats for 10 days. A significant change of epididymal weight has been observed after both treatments, from the lowest dose used. Epididymal dihydrotestosterone concentrations were significantly increased in flutamide- or casodex-treated rats, while epididymal 3 alpha-androstanediol concentrations were affected only after flutamide administration, suggesting a differential effect on androgen metabolism between both antiandrogens. Ornithine decarboxylase (ODC) activity was significantly decreased by flutamide, and to a lesser extent by casodex. Antiandrogen administration resulted in a significant decrease in epididymal content of the polyamines putrescine, spermidine, and spermine. A slight but significant decrease in putrescine and spermidine concentrations, but not in spermine, was observed after flutamide treatment. However, casodex had no effects on polyamines levels. A decrease in putrescine concentration was detected only when ODC activity fell to rather low levels. Interference of the antiandrogens with the biological action of androgens on ODC activity was clearly seen in immature male rats. Therefore, both epididymal growth and differentiation, in correlation to ODC activity, would be severely affected at an early period of sexual development, such as prepuberty.