Eleven patients with arteriographically confirmed coronary artery disease and normal arterial oxygen saturation were studied. The angina threshold was determined first by successive elevation of heart rate at increments of 10 and 5 beats/min by right atrial pacing while the patients were breathing air via a close-fitting mask. The mask was then connected to an oxygen tank without the patient's knowledge. After 5 min of oxygen breathing, the heart rate was again raised to the previously determined threshold level. In nine of 11 patients angina did not recur. The absence of angina was associated with improvement in myocardial lactate extraction from −17 ± 15 to +18 ± 10% (P< 0.025), in S-T abnormalities in six of seven patients, and in pulsus alternans in three of five patients.The pacing rate was then raised at increments of 5 beats/min until angina recurred. With oxygen breathing, angina developed at higher pacing rates (129 ± 7 beats/min with air and 137 ± 6 beats/min with oxygen, on the average;P< 0.005), at higher rate-pressure product (18.0 ± 0.8 and 19.5 ± 0.9 × 103mm Hg/min, respectively;P< 0.01), and at higher left ventricular oxygen consumption (21.3 ± 1.1 and 24.6 ± 1.1 ml/min, respectively;P< 0.005). The results indicate that oxygen breathing permits the heart to do more work before coronary insufficiency develops.