The mechanism by which omega 3 fatty acids lower plasma triacylglycerol levels was investigated. Rats were fed fish oil, olive oil (10% fat by weight) or a nonpurified diet (4% fat by weight) for 15 days. Lipoprotein lipase was inhibited by intra-arterial administration of Triton WR 1339 to estimate hepatic triacylglycerol output. Rats fed the olive oil diet showed a higher rate of triacylglycerol formation than rats fed the omega 3 fatty acid diet or the low-fat diet. All three groups showed identical rates of removal from plasma of intraarterially administered artificial chylomicrons that had simultaneously been labeled with cholesteryl [1-14C]oleate and [9,10(n)-3H]triolein. Liver radioactivity and total fat content were lowest in rats fed the fish oil diet, indicating that omega 3 fatty acids were preferentially metabolized in liver. Chylomicrons obtained from donor rats fed either fish oil containing [14C]cholesterol or olive oil containing [3H]cholesterol were removed at similar rates when infused together intraarterially into recipient animals. A slower formation of plasma very low density lipoprotein triacylglycerols in rats fed fish oil is probably due to a faster rate of oxidation of the fatty acid chains in the liver resulting in decreased plasma triacylglycerol concentrations.