The administration of saturating doses of insulin to the glucose perfused, working rat heart acutely increased activity of the glucose transporter 4, GLUT 4, in the plasma membrane (equilibrating extracellular glucose and intracellular [glucose]), activated glycogen synthase (stimulating the rate of glycogen synthesis), and increased mitochondrial acetyl CoA production by the pyruvate dehydrogenase multienzyme complex. Unexpectedly, insulin increased cardiac hydraulic work but decreased net glycolytic flux and O2 consumption, improving net cardiac efficiency by 28%. These improvements in physiologic performance and metabolic efficiency resulted from reduction of the mitochondrial free [NAD+]/[NADH] and oxidation of mitochondrial [coenzyme Q]/[coenzyme QH2], increasing the energy of the proton gradient between cytosolic and mitochondrial phases and leading to a doubling of the cytosolic free [sigmaATP]/[sigmaADP][sigmaPi]. The acute metabolic effects of insulin were qualitatively duplicated by addition of a ratio of 4 mM D-beta-hydroxybutyrate and 1 mM acetoacetate, and the increase in the efficiency was the same as with addition of insulin. Addition of both insulin and ketones to the glucose perfusate increased the efficiency of cardiac hydraulic work by 35%. The ability of a physiologic ratio of ketone bodies to correct most of the metabolic defects of acute insulin deficiency suggests therapeutic roles for these natural substrates during periods of impaired cardiac performance and in insulin-resistant states.