Recently, cases of liver damage and liver tumors have been reported after treatment of prostate cancer patients with the antiandrogen cyproterone acetate (CPA). In rat liver, CPA initiates a wave of DNA synthesis that is accompanied by apoptosis. In apoptotic hepatocytes, a latent form of transforming growth factor β1 (TGF-β1) is detectable by immunohistochemistry. Injection of a single dose of TGF- β1 induces apoptosis in the liver of animals pretreated with CPA but has an insignificant effect in untreated animals. In this study, we show by Northern analysis that there is increased expression of TGF-β1 in the liver after CPA treatment. Detection of TGF-β1 with in situ hybridization showed that TGF-β1 was synthesized in the parenchymal cells. Time course and dose-response experiments performed 48 hours after the last application of CPA showed that apoptotic nuclei with chromatin condensed at the nuclear periphery (AN) were already visible 2 hours after injection (0.13%), and apoptotic bodies (ABs) increased 2 to 9 hours after the injection (from 1.28% to 6.67%) after 25 μg TGF-β1/kg. At 4.5 hours after injection, an induction of apoptosis could be detected with 0.25 microgram TGF-β1/kg and after the maximum dose (250 μg TGF-β1/kg) ANs (0.24%) and ABs (16.74%) were homogeneously distributed throughout the liver lobe. Irrespective of the dose or time after injection of TGF-β1, 82% of the ABs were localized within hepatocytes. Liver enzymes were detected in high amounts in the serum (eightfold elevation of glutamate dehydrogenase, fivefold elevation of alanine transaminase [ALT]) 7 hours after the first visible sign of apoptosis. After an additional 20 hours, the liver contained many necrotic figures. These results suggest that the combination of TGF-β1 expression coupled with a strikingly enhanced sensitivity to the induction of apoptosis could be responsible both for the liver damage and the development of liver tumors observed after treatment with CPA.