SummaryThe effect of carnitine on the uptake, oxidation, and esterification of circulating palmitate has been studied with the isolated perfused rat heart. At concentrations of 0.5 mM and 5.0 mM, carnitine depressed the extraction of palmitate by perfused heart throughout the 45-min perfusion period. There was no significant effect of perfusing carnitine on the oxidation of labeled palmitate to 14CO2, but this could be explained by the low level of carnitine uptake under the same experimental conditions. Despite this, both levels of carnitine resulted in an increased percentage of recovery of extracted palmitate in the esterified lipids of the heart. Among the various lipid fractions of cardiac tissue, 5 mM carnitine perfusion caused a decrease in the percentage of label in unesterified fatty acids and diglycerides, and a significant increase in label recovered as phospholipids and triglycerides. This apparent increase in esterification of labeled palmitate was confirmed by separation of the subcellular f...