Carnitine analogs containing one and/or two methyl substituents on the α-, β-, or γ-carbon were evaluated in isolated rat liver mitochondria for their effects on fatty acid oxidation. Their abilities to either support, in the absence of carnitine, or inhibit, in the presence of carnitine, carnitine-dependent fatty acid oxidation were determined by the conversion of radiolabeled [1-14C]palmitic acid to acid-soluble radiolabeled products. None of the methylcarnitine analogs were observed to be significant inhibitors of palmitate oxidation at concentrations (1.0 mM) up to ten times that for icarnitine. The two diastereomers of d,l-4-methylcarnitine, however, were able to support palmilate oxidation in the absence of carnitine, and rates were roughly 40% of that obtained with equimolar (0.1 mM) l-carnitine. © 1993.