The purpose of the present study was to investigate the effect of oxygen inhalation on the stressed (pacing-induced) ischemic myocardium. In 13 open-chest dogs, the left anterior descending coronary artery was partially occluded, the coronary vein accompanying the artery was cannulated for coronary venous blood sampling and a myocardial strain gauge arch was sutured on the myocardium perfused with the partially occluded coronary artery in order to measure the myocardial contractile force. The heart rate was increased by left atrial pacing to increase myocardial oxygen consumption. Respiration was instituted either with oxygen or with room air. Increase of the heart rate caused an increase of blood flow through the partially occluded coronary artery, an increase of the local myocardial oxygen consumption and no consistent change of the local myocardial contractile force both during air breathing and oxygen breathing, provided that the heart rate was less than 200. These values decreased if the heart rate was increased more than 200 both during air and oxygen breathings. Coronary venous oxygen content from the local coronary vein accompanying the partially occluded coronary artery was 2.7 ml per 100 ml on the average during air breathing, while it was elevated to 3.8 ml per 100 ml during oxygen breathing. Although, pacing caused a similar change on coronary blood flow or myocardial contractile force both during air and oxygen breathings, elevated coronary venous oxygen content during oxygen breathing might indicate that ischemic myocardium becomes more aerobic during oxygen breathing. this may be one of the reasons why patient with coronary artery disease can perform more work without complaining chest pain during oxygen breathing.