Hemodilution is used frequently for patients undergoing elective coronary bypass surgery but relatively little is known about the effects of hemodilution on acute myocardial infarction. Left ventricular infarctions were planimetered after acute coronary occlusion in dogs treated with and without hemodilution. The control group was maintained at a normal hematocrit (40-45) while the intervention group was hemodiluted with 6% dextran in saline 30 min after left anterior descending coronary artery occlusion to a hematocrit less than 50% of control (20). Systemic arterial and left atrial pressure were maintained at control levels as were arterial pO2 and pH. Mean infarct size in the control group was 20.72 +/- 1.38% (SEM) of the left ventricle while the hemodiluted group had a mean infarct size of 18.80 +/- 2.11% of the left ventricle. Reduction of red cell mass to less than 50% of normal does not increase the size of myocardial infarction after acute coronary artery occlusion suggesting that oxygen supply to ischemic areas is maintained if systemic hypoxia is prevented.