PREVIOUS work on intact animals in which arteriovenous differences across the myocardium were measured^1,2 have suggested that fatty acids are an important metabolite for the heart. No work has been reported on the isolated heart with radioisotopes which allow a separation of the uptake and fate of a particular substrate. This communication gives an account of the uptake and certain metabolic fates of fatty acid and glucose in the isolated, perfused heart. The results indicate the readiness with which the myocardium utilizes both glucose and fatty acids, and suggest that fatty acids are preferentially oxidized. The latter observation provides a possible explanation for the puzzling increase in cardiac glycogen known to occur with fasting or with untreated diabetes.