Copenhagen x Fisher F1 rats were implanted with the androgen-dependent Dunning R3327 prostatic adenocarcinoma. When the tumors had median volumes of ca 470 mm3, the rats were castrated and/or treated with 6-methylene-4-pregnene-3,20-dione (6MP) in different doses. Tumor growth inhibition occurred in all castrated and treated groups, with decrease in volume of the epithelial compartment in the intact group. Tumor volumes at the highest dose level of 6MP equalled those observed in the castrate group. Plasma levels of testosterone were within the normal range. The administration of 6MP surprisingly induced an increment of tumor blood flow in the castrate group. Also in castrated and testosterone-supplemented animals, 6MP induced a reduction of prostatic tumor growth. Through the castration-like effect on tumor growth, the use of 6MP may represent an attractive alternative to castration for treatment of androgen-responsive prostate cancer.