The relationship of O2 supply and demand was studied in a small region of a normally functioning left ventricle in 11 pentobarbital anesthetized open chest dogs. A small myocardial vein was catheterized for flow and venous O2 content measurements. An electromagnetic flow probe and screw-type occluder were placed on the supply artery. Arterial flow reductions produced a lesser decrease in venous outflow. This indicated that collateral channels were functional even with relatively small reduction in arterial flow. Even with complete arterial blockage, venous flow never fell below 13.7% of control. Supply had to be reduced to 80.8% of control before we could predict with 95% confidence that O2 demand would fall. Using this technique, it would be possible to test antianginal agents' actions on the O2 supply and demand relation in a region of flow restricted myocardium.