In a double-blind study, 10 patients with angina exercised until they developed angina, before and after breathing 50 ppm carbon monoxide on two mornings for 2 hours and compressed, purified air on two mornings for 2 hours. The mean carboxyhemoglobin levels were 1.07% before and 0.77% after breathing purified air (P < 0.001) and 1.03% before and 2.68% after breathing carbon monoxide (P < 0.001). The mean exercise times until onset of angina were 226.7 seconds before and 223.0 seconds after breathing purified air (not significantly different) and 224.3 seconds before and 187.6 seconds after breathing carbon monoxide (P < 0.001). At angina there was a significant decrease in systolic blood pressure, heart rate, and product of systolic blood pressure times heart rate after breathing carbon monoxide (P < 0.001) but not after breathing purified air, compared with control levels. Breathing 50 ppm carbon monoxide for 2 hours induced angina sooner and after less cardiac work.