This study examines the effects of graded reduction in regional coronary perfusion and changes in arterial oxygen concentration upon regional myocardial function and left ventricular function. In 14 open chest dogs, the distal left anterior descending coronary artery was cannulated and perfused at different pressures with blood equilibrated with either 21 or 95 percent oxygen, and regional function in the perfused myocardial segment was determined by use of the pressure-length loop. The mass of perfused left ventricular myocardium was determined by injection of sodium fluorescein at a constant pressure of 100 mm Hg. Regional function decreased minimally as coronary perfusion pressure and flow were reduced to 50 to 65 mm Hg and 25 to 55 ml/min per 100 g, respectively. When pressure and flow were reduced below these critical ranges, regional function decreased sharply. Changes in left ventricular function were comparable but of lesser magnitude. The relations between regional function or left ventricular function and coronary perfusion were not affected by changing arterial oxygen concentration, except at the low range of coronary perfusion pressures (22 plus or minus 6 mm Hg), at which pressure regional function was significantly lower (P less than 0.025) in the experiments with 95 percent oxygen. Thus, the sensitivity of myocardial performance to a decrease in effective coronary perfusion pressure and flow is remarkably dependent upon whether pressure and flow are above a critical level.