Expression of prohibition, a growing-regulatory protein, was immunohistochemically investigated in normal rat tissues, regenerating livers, and chemically induced preneoplastic and neoplastic hepatic lesions. Specific cell types including hepatocytes, striated and smooth muscle cells, salivary gland duct epithelial cells, chondrocytes, immature spermatocytes and oocytes were found to be positive. In regenerating livers, prohibitin protein disappeared as early as 3 h after two-thirds hepatectomy and returned to near the original level by 24 h, while its mRNA level did not markedly vary. The timing of the disappearance was coincident with the expression of c-myc, suggesting a relation to quiescent hepatocytes entering the cell cycle. However, no pronounced decrease was evident in the most hyperplastic hepatic nodules and hepatocellular carcinomas investigated. Examination of 9 rat hepatocellular carcinoma cell lines, 6 hyperplastic hepatic nodules and 5 hepatocellular carcinomas revealed a single case of a base substitution in prohibition cDNA, identified as a synonymous sense change. The observed abundant expression of prohibitin in quiescent hepatocytes and its rapid loss under conditions of regeneration indicate a growth-regulatory function, but our results do not suggest any critical role in rat hepatocarcinogenesis.