The relationship between extracellular albumin and non-esterified fatty acid (NEFA) concentrations and the rate of fatty acid oxidation was studied. The data were obtained from tests performed on a working rat heart. When NEFA concentration was increased the rate of fatty acid oxidation showed a saturation curve at a constant NEFA/albumin ratio. Keeping constant the albumin concentration, a rise in NEFA concentration resulted in a linear increase of fatty acid oxidation. No correlation, however, was found between fatty acid oxidation and the unbound fraction of fatty acids. These results suggest an albumin receptor mediated NEFA uptake. With this assumption the following rate law of the NEFA uptake was derived: UPT = UPT0 X [FA]/(Km + [ALB] ) where [FA] and [ALB] are the total NEFA- and albumin concentrations, UPT0 and Km are constants. The rates of oxidation computed with this equation show a very good congruence to the values obtained experimentally. The validity of the rate law is confirmed by the fact that it is shown to be in agreement to the results of other investigators.