Aortic and coronary blood flow and arterial pressure were measured at heart rates of 120-240 beats/min in unanesthetized dogs before and after chemical sympathectomy with 6 hydroxydopamine or after surgical sympathectomy. At rates of 120 beats/min, coronary flow was less and coronary resistance was higher in sympathectomized as compared to intact animals. At rapid heart rates, aortic flow was maintained well in each group and ability to augment coronary flow was comparable, i.e., maximum increases of 43, 40, and 46 in intact, chemically and surgically sympathectomized groups, respectively. As heart rate was increased, propranolol did not modify aortic or coronary flow adjustments in sympathectomized animals, but in intact animals produced greater reductions in aortic flow and prevented augmentation of coronary flow at rates above 210 beats/min. Cardiac sympathectomy reduces coronary blood flow but does not prevent maintenance of aortic flow or augmentation of coronary flow at maximum rates produced by ventricular pacing. Reduction of coronary flow at rapid rates after beta blockade is dependent on intact cardiac sympathetic nerves.