Coronary reperfusion of ischemic myocardium may be beneficial but is highly dependent upon occlusion and reperfusion times. To study the effects of early reperfusion on ischemic myocardium, 24 open chest pigs underwent coronary occlusion; one group was occluded for 40 min, and the other was occluded for 30 min followed by 10 min of reperfusion. Left ventricular wall thickness during systole and diastole was determined by ultrasound. Mitochondrial energy production and calcium content were evaluated from ischemic and nonischemic areas. Results showed: There was an absence of systolic thickness, a slight decrease of diastolic thickness from baseline, and a decrease in energy production in the ischemic myocardium. Reperfusion resulted in a diverse pattern of systolic and diastolic wall thickness in the ischemic area and a variable Ca2+ accumulation and mitochondrial ATP production. The variability of myocardial Ca2+ accumulation in the ischemic reperfused group correlated inversely with mitochondrial ATP production (r = -0.94) and directly with diastolic wall thickness (r = 0.65). Similarly, calcium accumulation, ATP production, and diastolic wall thickness correlated with mean blood pressure during reperfusion. These results suggest that many factors including individual characteristics of the animal and experimental conditions such as the level of blood pressure and the degree of calcium accumulation may determine outcome of reperfusion even in as brief a period as 10 min.