Hearts from three species of fish with varying myoglobin content were perfused with stepwise changes in input perfusate PO2 from approximately 160 to 10 mmHg. Flow through the heart, rate of contraction, and afterload were kept constant. This standardized stroke volume and bulk flow of perfusate to the myocytes since these hearts are nourished by the fluid in the ventricular lumen. In some cases NaNO2 was added to the perfusion medium to decrease existing levels of functional myoglobin. Myoglobin-rich hearts were able to extract a constant amount of oxygen until perfusate PO2 had fallen below 80 mmHg. At this point oxygen uptake began to decline. These hearts consumed oxygen until input PO2 was 10 mmHg or less. When normoxic conditions were restored the myoglobin-rich hearts showed complete recovery. Performance was maintained at a constant level over the entire range of input PO2. Myoglobin-poor hearts and nitrite-treated hearts were unable to sustain constant levels of oxygen consumption in the face of a declining perfusate PO2. These hearts were unable to extract oxygen from the medium and failed at perfusate PO2's of 40 mmHg for naturally myoglobin-poor hearts and 30 mmHg for nitrite-treated hearts. Half-maximal oxygen consumptions were attained by myoglobin-rich hearts at lower input PO2's than either myoglobin-poor or nitrite-treated hearts. The impact of myoglobin in intact heart is apparent at relatively high extracellular PO2's (40-80 mmHg) in this model system.