1. The metabolism of palmitate and especially of erucate was studied in hepatocytes isolated from rats fed for 3 weeks a diet containing peanut oil (diet, 1), rapeseed oil (diet 2) and partially hydrogenated marine oil (diet 3). 2. The metabolism of palmitate was not significantly influenced by the diet. The rapeseed oil diet caused 1.4 fold and 1.3 fold increase and marine oil diet 3 fold and 2.2 fold increase in the oxidation and chain-shortening respectively of [14-14C]erucic acid in isolated hepatocytes. 3. Cyanide and antimycin A did not inhibit the chain-shortening of erucate in liver cells of rats fed rapeseed oil and peanut oil. The high capacity of the chain-shortening system in hepatocytes of marine oil-fed rats was partially inhibited. 4. Inhibition of the transfer of fatty acids into the mitochondria by lowering the intracellular carnitine concentration and/or by addition of (+)-decanoyl-carnitine resulted in a very pronounced apparent stimulation of the chain-shortening of erucic acid. It is suggested that the chain-shortening system may be virtually independent of the mitochondria, unless the availability of the extramitochondria NAD+ and/or NADP+ is rate-limiting under conditions of extremely low redox potential of the mitochondria. 5. Feeding marine oil or rapeseed oil to the rats induced a 30% increase in catalase activity, a 25--30% increase in urate oxidase activity and a 50% increase in the total CoA in the liver compared to rats fed peanut oil. 6. It is suggested that the increased metabolism of erucate in hepatocytes of marine oil and rapeseed oil-fed rats may be due to the increase in ther peroxisomal beta-oxidation.