The time required for myocardial oxygen utilization to recover from 12 minutes of warm ischemia was studied in 15 sheep. Five control animals had 42 minutes of cardiopulmonary bypass at 37 degrees C with the heart beating and vented. In 10 experimental animals, the aorta was clamped for 12 minutes during bypass with the heart vented, and the animals were perfused 30 additional minutes after removal of the clamp. In both groups, measurements of left ventricular coronary arterial blood flow, oxygen consumption (LVO2), peak systolic pressure, circumferential systolic stress integral (CSI), and pressure volume area (PVA) were made 30 minutes after and hourly for 5 more hours after cardiopulmonary bypass ended. Reversible ischemia significantly increases the relationship between LVO2/PVA and LVO2/CSI 1 hour after the clamp is released, but the relationships return to prebypass values by 2 hours. Ischemia significantly decreases end-systolic elastance, which remains depressed thereafter. Cardiopulmonary bypass without myocardial ischemia significantly decreases total ventricular mechanical energy output and oxygen utilization efficiency of the working sheep heart but does not significantly alter end-systolic elastance or the LVO2/PVA and LVO2/CSI relationships. Twelve minutes of warm ischemia impairs oxygen utilization efficiency in the reperfused sheep heart for more than 60 minutes but less than 120 minutes.