We evaluated hyperthermic influences on ischemic hearts by comparing two groups of intact working swine hearts (n = 20) made globally ischemic. Heart muscle temperature was selectively increased from 37.5 +/- 0.3 degrees C to 39.7 +/- 0.3 degrees C in one group (n = 11) by warming the coronary perfusate. Ischemia in normothermic hearts significantly (P less than 0.05) decreased mechanical function (as reflected by increases in left ventricular end-diastolic pressure [LVEDP]), myocardial oxygen consumption (MVO2), glucose uptake, glycolytic flux, free fatty acid (FFA) uptake and oxidation, and tissue stores of high energy phosphates. Hearing in ischemic hearts further depressed mechanical function at similar reductions in coronary flow and MVO2. Glucose uptake was terminally increased over normothermic values (329 vs. 221 mumol/hr per g) as was glycolytic metabolism, FFA uptake (26 vs. 17 mumol/hr per g), and FFA oxidation (21 vs. 11 mumol/hr per g). However, these changes were not translated into increased energy stores of tissue creatine phosphate and ATP. Thus, in ischemic hearts, hyperthermia neither prevented the development of mechanical deterioration nor improved oxidative phosphorylation despite increases in metabolic substrate utilization. These data suggest that in experimental global ischemia heat is an added energy drain in already burdened myocardium.