Testosterone-induced DNA synthesis in cultured rat ventral prostate was evaluated as an in vitro model for screening the direct effects of anticancer agents on prostatic growth. Optimal conditions for this bioassay, particularly the concentration of testosterone, were established using the inhibitory effects of cyproterone acetate on testosterone-induced 125iododeoxyuridine (I-UdR) uptake as an index of DNA synthesis inhibition. Using 4 X 10(-7) M testosterone, only the highest concentration (X 10(-5) M) of cyproterone acetate inhibited I-UdR uptake and histological observations indicated that this was due to a non-specific cytotoxic effect. In contrast, cyproterone acetate had a dose-dependent inhibitory effect on the proliferative response to 4 X 10(-9) M testosterone. Cyproterone acetate (4 X 10(-7) M) combined with 4 X 10(-9) M testosterone exerted an inhibitory effect on I-UdR uptake and caused epithelial atrophy indicative of androgen deprivation. The results are consistent with similar in vivo studies and confirm the hypothesis that cyproterone acetate acts by directly antagonising testosterone action at the target tissue level. Thus, this in vitro method provides a useful model for screening the direct effects of antiprostatic drugs.