The pro-peptide of transforming growth factor a (proTGFa) was recently found in hepatocyte nuclei preparing for DNA replication, which suggests a role of nuclear proTGFa for mitogenic signalling. This study investigates whether the nuclear occurrence of the propeptide is involved in the altered growth regulation of (pre)malignant hepatocytes. In human hepatocarcinogenesis, the incidence of proTGFa-positive and replicating nuclei gradually increased from normal liver, to dysplastic nodules, to hepatocellular carcinoma. ProTGFa-positive nuclei almost always were in DNA synthesis. Also, in rat hepatocarcinogenesis, proTGFa-positive nuclei occurred in (pre)malignant hepatocytes at significantly higher incidences than in unaltered hepatocytes. For functional studies unaltered (GSTp À ) and premalignant (GSTp þ ) rat hepatocytes were isolated by collagenase perfusion and cultivated. Again, DNA synthesis occurred almost exclusively in proTGFa-positive nuclei. GSTp þ hepatocytes showed an B3-fold higher frequency of proTGFa-positive nuclei and DNA replication than GSTp À cells. Treatment of cultures with the mitogen cyproterone acetate (CPA) elevated the incidence of proTGFa-positive nuclei and DNA synthesis in parallel. Conversely, transforming growth factor b1 (TGFb1) lowered both. These effects of CPA and TGFb1 were significantly more pronounced in GSTp þ than in GSTp À hepatocytes. In conclusion, nuclear translocation of proTGFa increases in the course of hepatocarcinogenesis and appears to be involved in the inherent growth advantage of (pre)malignant hepatocytes.