Isolated rat hepatocytes were exposed to increasing concentrations of ethanol. During exposure of cells to ethanol a moderate but significant modification in the level of hepatic PKC c-isoforms has been observed. The ethanol-induced effect on liver protein kinase C was reversed by 4-methylpyrazole, an inhibitor of alcohol dehydrogenase, indicating that the conversion of ethanol to acetaldehyde may be involved in the enzyme inactivation. The involvement of the alcohol metabolite in PKC modifications was confirmed by the exposure of hepatocytes or partially purified liver enzyme to acetaldehyde concentrations of pathological interest.