In the presence of regional myocardial ischemia, a 20% decrease in systemic arterial pressure following nitroprusside caused a 25% decrease in coronary perfusion pressure in animals with normal left ventricular end-diastolic pressures. This pressure decrement resulted in a significant decrease in the shortening of the regionally ischemic segment during the ejection phase of systole. A comparable arterial pressure drop of 21% with nitroprusside infusion during ischemia in the animals with elevated diastolic pressures caused a similar 28% decrease in coronary perfusion pressure, but resulted in a simultaneous increase in regional shortening. For the entire group there was no significant change in stroke volume. Even in the 11 animals where stroke volume increased, systolic regional shortening increased in only 4. An increase in stroke volume cannot be used to infer a parallel increase in the performance of a regionally ischemic segment. Nitroprusside appears to improve regional performance only in the presence of severe failure.