1. Changes in the activities of several enzymes involved in mitochondrial fatty acid oxidation were measured in the livers of developing rats between late foetal life and maturity. The enzymes studied are medium-and long-chain ATP-dependent acyl-CoA synthetases of the outer mitochondrial membrane and matrix, GTP-dependent acyl-CoA synthetase, camitine acyltransferase, enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase, general 3-oxoacyl-CoA thiolase and acetoacetyl-CoA thiolase. Developmental changes in the activity of cytoplasmic acetoacetyl-CoA thiolase were also investigated. 2. The activities of all the mitochondrial enzymes studied increase markedly at birth and, except for enoyl-CoA hydratase, decrease at weaning. However, all the developmental patterns other than those of enoyl-CoA hydratase and acetoacetyl-CoA thiolase exhibit a fall followed by a rise in enzyme activity during the mid-suckling period. 3. The cytoplasmic acetoacetyl-CoA thiolase activity is low throughout suckling, but high before birth and after weaning. 4. The results are discussed in relation to changes in fatty acid supply to the liver and hormonal changes occurring during development.