Different substrate mixtures were investigated for their effect on energy metabolism using sections of the rat renal cortex. Simultaneous determination of adenine nucleotide concentrations and determination of the damage quotient of oxidative phosphorylation proved to be appropriate parameters for selecting substrate mixtures that have a favorable effect on energy metabolism. A mixture of albumin (1 mM) and octanoate (5.6 mM) with electrolytes proved to be adequate. The extent of oxygen consumption (60%) and 14CO2 formation (75%) argues in favor of the metabolization of this mixture; a damage quotient of 23% and general constancy of the concentration of high-energy compounds render prospective their testing in animal experiments. Addition of dicarbosylic acids increase the antimycin A resistant oxygen consumption without any energy conservation being demonstrable. Therefore, these substrates should not be used for conservation.