Twenty patients with fixed coronary artery obstruction were studied during rapid atrial pacing and methoxamine infusion. During pacing to heart rates of 142 ± 4 (mean ± SEM) beats per minute coronary sinus flow increased from 108 ± 8 to 187 4 15 cc/min and myocardial oxygen consumption increased by + 80 + 11%. During methoxamine infusion that raised arterial systolic pressure to 196 i 5 mm Hg, similar increases in coronary sinus flow (to 179 i 13 cc/min) and myocardial oxygen consumption (+ 77 i 12%) occurred. Chest pain and ischemic ST segment changes developed in 17 and 14 patients respectively during atrial pacing, an incidence significantly greater (P < 0.05) than during infusion of methoxamine (6 and 3 patients). Myocardial lactate extraction which averaged 26 i 4% during control was decreased to 10 + 8% during pacing and to 24 ± 7% during methoxamine; the difference between decreases was not significant. The data show that at similar increases in myocardial oxygen consumption stress of increased heart rate results in more myocardial ischemia than stress of increased afterload.