1. Carnitine and carnitine palmitoyltransferase are active in the transfer of fatty acids into the mitochondria for oxidation. Very long chain fatty acids (C22) are poorly oxidized by mitochondria. Lack of carnitine or overloading with C22 fatty acids leads to lipidosis in heart and other tissues. 2. The oxidation of fatty acids (including C22 fatty acids) in the peroxisomes is not dependent on carnitine. However, carnitine acetyltransferase and carnitine medium chain acyltransferase are presumably auxiliary enzymes in the oxidation of acetyl-CoA and shortened fatty acids formed in the peroxisomes. 3. Branched-chain acylcarnitines may be formed in the mitochondria from branched-chain amino acids. They are also metabolized in the mitochondria. When formed in large amounts, they are released into the circulation and urine by the liver and kidney. 4. The mechanisms leading to secondary carnitine deficiency because of branched-chain acylcarnitine formation in metabolic disturbances are discussed.