This study was conducted to determine whether or not a low concentration of carboxyhemoglobin influences the extent and severity of myocardial ischemia caused by coronary ligation. In 10 dogs, electrograms were recorded from 6 epicardial electrodes mounted on the anterior surface of the left ventricle and distributed over the area normally perfused by the lighted branch of the left anterior descending coronary artery. The magnitude of ST segment elevation of the 6 sites in each animal was determined for 15 min after ligation. This elevation was used as an index of the presence and severity of myocardial ischemic injury. Ligation alone increased sigma ST elevation, summed from 6 sites, from 2.06 +/- 0.34 mV (SEM) to 24.89 +/- 2.14 mV (SEM). Carbon monoxide inhaled prior to ligation increased the severity and extent of ischemic injury and the magnitude of ST segment elevation in the area peripheral to the ischemic area more than did ligation alone. These changes occurred without elevation of heart rate or arterial pressure. It was concluded that a low background concentration of carboxyhemoglobin at the time of ligation increased the extent and severity of myocardial ischemic injury.