The oxidation of leucine, isoleucine and the corresponding 2-ketoacids was studied in perfused hindquarter and liver of rat. The metabolites from these compounds released into the perfusate were identified by anion exchange chromatography and gas chromatography/mass spectrometry. Muscle perfused in the presence of leucine accumulated ketoleucine (2-ketoisocaproate) and isovalerate, while perfusion with isoleucine resulted in ketoisoleucine (2-keto-3-methylvalerate) and 2-methylbutyrate. Octanoate stimulated the oxidation of the branched-chain amino acids. The fatty acid did not change the amount of branched-chain 2-keto acids released into the perfusate, but increased the output from the muscle of the oxidation products isovalerate (from leucine) and 2-methylbutyrate (from isoleucine). Rat liver perfused with ketoleucine released mainly ketone bodies and small quantities of 3-hydroxyisovalerate while ketoisoleucine resulted mainly in release of lactate and ketone bodies. Accumulation of metabolites from the branched-chain amino acids is discussed in relation to the metabolism of the amino acids in skeletal muscle and liver of rat.