Conjugated linoleic acid (CLA) is known to modulate lipid and lipoprotein metabolism. We examined the effects of dietary CLA (1 and 2%) on hepatic microsomal triacylglycerol transfer protein (MTP) activity known to be essential for the assembly/secretion of the apolipoprotein B-containing lipoproteins, and lipid concentrations of liver and plasma in OLETF obese rats. A significant reduction in hepatic MTP activity in CLA-fed rats in comparison to rats fed a linoleic acid (LA) diet was noted. Hepatic and serum triacylglycerol (TAG) concentrations were lowered in the CLA groups as compared to the control LA group. Decreased serum insulin and leptin levels were seen in the CLA groups. These results indicate that dietary CLA reduces both hepatic MTP activity and TAG content, and therefore reduces hepatic lipoprotein assembly and secretion. © 2003, Japan Oil Chemists' Society. All rights reserved.