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The cardiac metabolism of C 14 -labeled nucleosides and bases was investigated in isolated rabbit hearts perfused with a glucose-electrolyte medium. Records of contractile force, heart rate and coronary drainage were taken at intervals before, during, and after perfusion with a labeled compound. Contractile force was increased in the presence of inosine, uridine, guanosine, and hypoxanthine; and decreased in the presence of adenine. Determination of radioactivity of aliquots of perfusate during C 14 perfusion and washout periods showed generally constant uptake of the labeled compounds by the hearts, after an initial dilution by tissue water, and efficient washout of extracellular label before preparing myocardial extracts. Recirculation of inosine-C 14 was accompanied by incorporation of radioactivity into cardiac nucleotides, chiefly adenine nucleotides but also inosinic acid (IMP) and guanine nucleotides. Labeled hypoxanthine was recovered from both heart and perfusate. Recirculation of uridine-C 14 was accompanied by incorporation of radioactivity into cardiac uridine nucleotides, and the appearance of labeled uracil in the perfusate. In both groups of experiments, the ratio of labeled base to nucleoside in cardiac extracts exceeded the ratio in perfusates, indicating unequal cellular retention of base and nucleoside. Perfusion with guanosine-C 14 led to limited incorporation of C 14 into guanine nucleotides without interconversion to other cardiac nucleotides. Recirculation of C 14 -labeled hypoxanthine or adenine was accompanied by incorporation of radioactivity into cardiac purine nucleotides. Equivalent label distribution was found among adenosine monophosphate, adenosine diphosphate, and adenosine triphosphate, indicating intracellular equilibrium among these compounds. Conversion of labeled adenine to hypoxanthine occurred in the perfusate of the adenine-C 14 experiments. The cardiac metabolism of the perfused nucleosides and bases followed generally predictable sequences, and did not suggest a mechanism for the marked inotropic activity of these compounds.