The effect of heart rate on the amount and distribution of collateral blood flow was determine in open-chested dogs 1 h after coronary artery ligation. Flows to ischemic and nonischemic regions of left ventricle were measured with 7- to 10- mum diam radioactive microspheres during base-line conditions (118 +/- 6 beats/min) and again during atrial pacing at rates 20 and 40% above control (141 +/- 7 and 165 +/- 9 beats/min). During pacing aortic and left atrial pressures and cardiac output did not change significantly, whereas ST segment elevation in epicardial electrograms increased markedly. In nonischemic myocardium, mean flow increased approximately in proportion to the increase in rate, but subepicardial (EPI) flow increased somewhat more than subendocardial (ENDO) flow. In ischemic myocardium, overall flow did not change significantly, but a redistribution from ENDO to EPO was seen. At the faster rate ENDO flow fell 25% (P less than 0.02), EPI flow increased slightly, and ENDO/EPI fell in 8/9 animals (mean 0.54-0.43, P less than 0.01). The ENDO/EPI maldistribution present in ischemic muscle is thus accentuated by tachycardia; this may account for part of the harmful effect of tachycardia in acute myocardial infarction and may help explain the disproportionate ENDO ischemia seen in angina pectoris.