Carnitine is an essential factor in long-chain fatty acid oxidation. Carnitine acts as a carrier of fatty acyl groups from the cytoplasm to the mitochondrion. Long-chain acyl-CoA derivatives do not penetrate the mitochondrial inner membrane. Carnitine palmitoyltransferase A (CPT-A), located on the external surface of the inner membrane, catalyzes the conversion of cytoplasmic long-chain acyl-CoA and carnitine into acylcarnitine. The acylcarnitine is reconverted to intramitochondrial acyl-CoA by the action of carnitine palmitoyltransferase B located in the inner membrane. Now, the acyl-CoA is available for beta-oxidation in the matrix. An inner membrane carnitine-acylcarnitine translocase exchanges carnitine and acylcarnitine across the inner membrane but its role is long-chain acyl transfer has not been established. The tissue concentration of carnitine is important; liver carnitine is correlated with the rate of hepatic ketoacid production. In liver, malonyl-CoA, an intermediate in fatty acid synthesis, is proposed to regulate the activity of CPT-A. Studies using various purified transferases have not provided an answer to the question of whether the two activities expressed in mitochondria are separate enzymes. The absence of only CPT-A activity in isolated skeletal muscle mitochondria obtained from a patient with a lipid-storage myopathy suggests two separate activities.