N,N-Dimethylformamide (DMF) is an organic solvent extensively used in industries such as synthetic leather, fibers and films, and induces liver toxicity and carcinogenesis. Despite a series of experimental and clinical reports on DMF-induced liver failure, the mechanism of toxicity is yet unclear. This study investigated whether DMF in combination with a low dose of hepatotoxicant enhances hepatotoxicity, and if so, on what mechanistic basis. Treatment of rats with either DMF (50-500mg/kg/day, for 3 days) or a single low dose of CCl(4) (0.2ml/kg) alone caused small increases in plasma transaminases and lactate dehydrogenase activities. However, combinatorial treatment of DMF with CCl(4) markedly increased blood biochemical changes. Histopathology confirmed the synergism in hepatotoxicity. Moreover, DMF+CCl(4) caused PARP cleavage and caspase-3 activation, but decreased the level of Bcl-xL, all of which confirmed apoptosis of hepatocytes. Consistently, DMF+CCl(4) treatment markedly increased lipid peroxidation. By contrast, treatment of DMF in combination with lipopolysaccharide, acetaminophen or d-galactosamine caused no enhanced hepatotoxicity. Given the link between endoplasmic reticulum (ER) dysfunction and cell death, ER stress response was monitored after DMF and/or CCl(4) treatment. Whereas either DMF or CCl(4) treatment alone marginally changed the expression levels of glucose-regulated protein 78 and 94 and phosphorylated PKR-like ER-localized eIF2alpha kinase, concomitant treatment with DMF and CCl(4) synergistically induced them with increases in glucose-regulated protein 78 and C/EBP homologous protein mRNAs. Our results demonstrate that DMF treatment in combination with CCl(4) synergistically increases hepatocyte death, which may be associated with the induction of severe ER stress.