The purpose of this study was to determine the effect of graded doses of norepinephrine on the regional O2 supply/consumption ratio in ischemic rabbit hearts. Open chested, anesthetized rabbits were used and regional flows, O2 extraction, consumption and O2 supply/consumption ratios were determined before and 1 h after occlusion of the left anterior descending coronary artery in controls and animals given 0.1, 1.0 and 10 micrograms/kg per min norepinephrine (NE). After occlusion in controls, mean myocardial blood flow decreased 40% in the occluded region. Blood flow was also depressed in the occluded region for all NE doses compared to their own preocclusion values, but was higher in these regions compared to control animals. O2 extraction was higher in the occluded region compared to the nonoccluded region for all groups; however these values were lower in the NE groups compared to controls. NE increased O2 consumption in the occluded and nonoccluded regions compared to control group values. The O2 supply/consumption ratio was depressed in the occluded region in all groups compared to the nonoccluded region, though no differences were seen between NE and control groups. Thus, increases in the blood flow to the occluded region were proportional to the increases in O2 consumption with infusion of NE, indicating that a reserve exists and can be utilized with NE.