The aim of this study was to determine the effect of low-pressure and high-pressure reperfusion, with and without ventricular fibrillation, on the recovery of hypertrophic and normal hearts after hypothermic cardioplegia. Fourteen hearts rendered hypertrophic by valvular aortic stenosis and 18 normal canine hearts were subjected to 1 hour of cardioplegic arrest at 28 degrees C during cardiopulmonary bypass. Each heart was then reperfused at a coronary pressure of either 40 mm Hg (low) or 80 mm Hg (high), initially in the empty beating state and then during ventricular fibrillation. Low-pressure reperfusion produced left ventricular subendocardial ischemia in hypertrophic and in normal hearts, shown by marked depression of subendocardial blood flow, myocardial pH, and myocardial oxygen consumption. In hypertrophic hearts the ischemia was more severe and resulted in a persistent depression of left ventricular function and myocardial oxygen consumption even when coronary pressure was returned to normal levels. High-pressure reperfusion was associated with rapid and complete recovery of myocardial metabolism and function in hypertrophic and in normal hearts. During low-pressure reperfusion, ventricular fibrillation exacerbated ischemia in hypertrophic and in normal hearts. During high-pressure reperfusion, a short period of ventricular fibrillation produced no adverse effects either in hypertrophic or in normal hearts. We conclude that low-pressure reperfusion produces subendocardial ischemia in normal and in hypertrophic hearts even in the empty beating state; in hypertrophic hearts it also impairs recovery of myocardial metabolism and function. The adverse effects of low-pressure reperfusion are exacerbated by ventricular fibrillation.