We have recently shown that feeding apolipoprotein E knockout (apoE^-/-) mice a high-fat diet for 6 months accelerates progression of coronary disease and induces changes to cardiac metabolism and function. These changes could be due to coronary disease and/or direct effect of high-fat feeding. Wild-type mice are resistant to coronary disease (Breslow 1996) and therefore, when fed a high-fat diet, provide an excellent model to investigate the effect of high-fat diet on cardiac function, independent of coronary disease. Male C57/129 wild-type mice were weaned onto a normal rodent diet, and then at approximately 8 weeks old, animals were either switched onto high fat, Western-type diet (21% fat; 0.15% cholesterol) or were maintained on normal rodent diet. Animals were fed their respective diets for 6 months. Histological examination of hearts confirmed absence of coronary disease in both groups. Isolated hearts from both groups were used to measure cardiac metabolites and function. Isolated myocytes were used to measure mitochondrial flux (NAD⁺/NADH), contractile function and calcium transients. Mice fed high-fat diet had significantly increased levels of cardiac lactate (from 52 ± 4 to 76 ± 6 nmol/mg protein) and decreased glycogen content (from 0.09 ± 0.005 to 0.06 ± 0.008 mg/g wet weight) but had similar levels of energy rich phosphates. Evidence of metabolic stress in high-fat fed hearts was confirmed in isolated perfused myocytes which showed increased NAD⁺/NADH ratio (0.34 ± 0.02 vs. 0.29 ± 0.01). These metabolic differences did not alter functional characteristics of isolated perfused myocytes and hearts. However the rate of rise and decline of calcium transients were slower in high-fat fed myocytes. Intact perfused hearts from high-fat fed animals were significantly more vulnerable to reperfusion injury than those from animals fed normal diet. In conclusion, high-fat diet has profound metabolic effects on the myocardium that is independent of coronary disease. These changes appear to alter vulnerability to reperfusion injury.