These experiments were performed in order to ascertain whether activation of coronary vascular adrenergic receptors could change the time course of the coronary vasodilatation accompanying increases in myocardial metabolic activity. The left common coronary arteries of dog hearts were perfused in situ with blood at constant flow. Coronary perfusion pressure and coronary sinus blood 02 content were continuously monitored. Norepinephrine was infused into the coronary artery at 0.5 to 5 microgram.min-1. and isoprenaline at 0.5 to 2 microgram.min-1. The amplitude of the vascular resistance change was less with norepinephrine than with isoprenaline infusion for a similar change in oxygen consumption. The time course of coronary vascular resistance, after correction for the effects of vascular transit, lagged significantly behind the time course of coronary sinus 02 content in the case of norepinephrine infusion. On the average, no lag was observed with isoprenaline infusion. It is concluded that stimulation of coronary vascular adrenergic receptors can alter the time course and the magnitude of the coronary vascular response to increases in myocardial metabolic activity resulting from myocardial beta-receptor stimulation.