Abstract: A comparative study of the effects of hemodilution with dextran and fluorocarbon emulsion-Ⅲ (FCE-Ⅲ, 20% perfluorotripropylamine) on the changes in blood viscosity and collateral flow following coronary occlusion was performed on 14 anesthetized open-chest dogs. The results showed that, after hemodilution with FCE-Ⅲ, the blood viscosity was decreased by 41%, erythrocyte aggregating index decreased by 26.9% and collateral blood flow and supply/demand ratio increased significantly. These changes were the same as those resulted from hemodilution with dextran. In addition, hemodilution with FCE-Ⅲ exclusively increased PO₂ and dissolved oxygen and so increased oxygen supply to the ischemic myocardium.