A procedure has been developed for the quantitative determination of the extractable proteins of the rat heart. Utilizing extraction with a sucrose-versene solution and employing differential centrifugation, followed by extraction with a phosphate-pyrophosphate buffer, low-ionic-strength extractives, a mitochondrial fraction, and high-ionic-strength extractives have been obtained. Electrophoretic analysis has shown five subfractions to be present in the low-ionic-strength extractives and three sub-fractions in the high-ionic-strength extractives.