Carbon tetrachloride (CCl4) interferes with triglyceride secretion and causes steatosis, fibrosis and necrosis. In mice, CCl4 decreased plasma triglyceride-rich lipoproteins, increased cellular lipids, and reduced microsomal triglyceride transfer protein (MTP) without diminishing mRNA levels. Similarly, CCl4 decreased apoB-lipoprotein production and MTP activity but had no effect on mRNA levels in primary enterocytes, colon carcinoma and hepatoma cells. CCl4 did not affect MTP synthesis, but induced post-translational degradation involving ubiquitinylation and proteasomes in McA-RH7777 cells. By contrast, MTP inhibitor increased cellular lipids without affecting MTP protein. MTP was covalently modified when cells were incubated with 14CCl4. This modification was prevented by the inhibition of P450 oxygenases indicating that CCl3· generated by these enzymes targets MTP for degradation. To determine if inhibition of proteolysis could prevent CCl4 toxicity, mice were fed with CCl4 with or without lactacystin. Lactacystin increased ubiquitinylated MTP and prevented lipid accumulation in tissues. Thus, CCl4 induces post-translational degradation without affecting lipid transfer activity, whereas MTP antagonist inhibits lipid transfer activity without causing its destruction. These studies identify MTP as a major target of CCl4 and its degradation as a novel mechanism involved in the onset of steatosis, and suggest that inhibition of proteolysis may prevent some forms of steatosis.