In the present study the effects of some C18 fatty acids on hepatic fatty acid metabolism have been compared. Male rats were fed cholesterol-free diets containing either C18:0, C18:1cis or C18:1trans isomers as the variables. In accordance with previous work, oleic acid in the diet caused an increase in cholesterol concentration in the liver and in the lipoprotein fraction of density (d; kg/l)<1·006. Oleic acid also reduced the triacylglycerol:cholesterol value in this fraction. Surprisingly, the C18:1trans isomers diet induced a decrease in the amount of cholesterol in total plasma as well as in the 1·019<d<1·063 lipoprotein fraction. Both oleic acid and C18:1trans isomers increased the concentration of triacylglycerols in the liver. The two C18:1 fatty acids differently influenced the hepatic activities of carnitine palmitoyltransferase-I and 3-hydroxy-acyl-CoA dehydrogenase; both enzymes were inhibited by C18:1trans isomers, while no change was induced by oleic acid. The activity of the citrate carrier was lower in the oleic acid- and C18:1trans isomers-fed rats, when compared with the rats fed stearic acid. No diet effects were seen for the activities of acetyl-CoA carboxylase, fatty acid synthase, diacylglycerol acyltransferase, citrate synthase and phosphofructokinase. The results are interpreted in that oleic acid raised liver triacylglycerol by reducing the secretion of it with the d<1·006 lipoprotein fraction whereas the C18:1trans isomers enhanced liver triacylglycerol by lowering the hepatic oxidation of fatty acids.