Explants of the ventral prostate of adult rats, were maintained in organ culture with various steroids and insulin. In addition to androgens glucocorticoids and insulin were necessary for the maximal stimulation of the explants as judged by measuring the incorporation of labeled precursors into DNA, RNA and protein and the formation of 14CO2 from 1FC-glucose by explants. The addition of cyproterone, a steroid with high antiandrogenic potency to the culture medium suppressed the testosterone action, but did not interfere withthe effects of insulin. These findings were taken as a proof of the direct and specific antiandrogenic action of cyproterone on the rat prostate.