The activity of dicarboxylyl-CoA synthetase previously reported in rat liver was detected, in the presence of added detergents, in rat skeletal muscle. In both tissues, carnitine dicarboxylyltransferase activities were recorded but their substrate chain length specificity was, however, different. In rat skeletal muscle, but not liver, a carnitine-dependent oxidation by intact mitochondria of dodecanedioyl-CoA was easily detected by the spectrophotometric measurement of the substrate-dependent ferricyanide reduction. The implications of the present data for the pathogenesis of disorders with excess urine dicarboxylylcarnitine esters are discussed.