Previous studies have demonstrated that certain high fat diets can induce peroxisomal fatty acid β-oxidation in rodent liver and that this may be due to their content oftrans22∶1 fatty acids. In this study we have examined the effects ofcisandtrans22∶1 fatty acids (erucic and brassidic) and oleic acid (18∶1) on palmitoyl-CoA oxidation, carnitine acetyltransferase and carnitine palmitoyltransferase activities in primary rat hepatocyte cultures. Brassidic and erucic acid and, to a lesser extent, oleic acid were cytotoxic to rat hepatocytes. However, at a concentration of 0.1 mM, brassidic acid produced small increases in palmitoyl-CoA oxidation and carnitine acetyltransferase activities in hepatocytes cultured 70 hr. Treatment of cells with 0.1 and 0.3 mM of either erucic or oleic acid had no effect on any of the enzymes me