We studied hepatotoxicity of dichloromethane using primary cultures of parenchymal cells (hepatocyte) from adult rat livers. The production of carbon monoxide from dichloromethane increased with time, the increased cell number, and the concentration of dichloromethane. However, the carbon monoxide production per hepatocyte decreased with increasing cell density. When dichloromethane was in a high concentration, the metabolism of dichloromethane to carbon monoxide was extensively depressed, total glutamic-oxaloacetic transaminase (GOT) and mitochondrial GOT (m-GOT) levels in the cultured medium were extensively elevated, and the cultured hepatocytes were destroyed by dichloromethane. A case of accidental exposure to dichloromethane which occurred in Japan was also considered, and it is suspected that the inhalation of dichloromethane vapors in a high concentration for many hours may cause hepatotoxicity.