Rats bearing the Dunning R3327H prostatic adenocarcinoma were castrated and supplemented with testosterone propionate. Some of the testosterone-supplemented rats were treated with medroxyprogesterone acetate (MPA) alone or in combination with epirubicin hydrochloride. During the treatment period of 4 weeks, the growth rate of the prostatic tumors was measured. The testosterone-supplemented rats showed a tumor growth rate similar to that found in intact control rats. It was found that MPA has an inhibitory effect on the tumor growth rate in castrated testosterone-supplemented animals. This effect was more pronounced if the MPA treatment was combined with epirubicin hydrochloride, where the tumor growth rate was slower than that found in rats castrated as the only treatment.