Using a computer-assisted working rat heart preparation, which allows continuous registration of the respiratory quotient, it was tested which parameters determine fatty acid oxidation in the myocardium. Supplying albumin and palmitate in different concentrations the rate of fatty acid oxidation was measured. The UFA concentrations were calculated using stepwise equilibrium constants. When keeping constant the NEFA/albumin ratio and raising total NEFA concentration, an increase in fatty acid oxidation was found showing a saturation curve. Increasing NEFA at constant albumin concentration, however, results in a linear increase in fatty acid oxidation. Keeping constant the total NEFA concentration elevation of albumin shows an inhibitory effect. These results suggest the existence of a receptor for albumin on heart cell surface, which mediates uptake of albumin-bound NEFA. An additional supply of glucose and lactate does not show any effect on these relations. Acetate and dichloroacetate, an activator of the pyruvate dehydrogenase, are found to be competitive inhibitors of fatty acid oxidation.