To examine the role of neural factors in the control of coronary vasoactivity in conscious animals, dogs were supplied with miniature pressure gauges in the aorta and left ventricle (to measure aortic and left ventricular pressures, respectively and with a flow probe on the left circumflex coronary artery (to measure coronary blood flow). The experiments were conducted several weeks after recovery from operation. Stimulation of the carotid chemoreceptor and pulmonary inflation elicited a biphasic reflex response. Initially, coronary vasodilation was observed; coronary blood flow tripled even after changes in metabolic factors were minimized by pretreatment with propranolol. A similar response occurred after a spontaneous deep breath. The coronary vasodilation could be blocked by alpha-adrenergic receptor blockade. The second phase of the response involved an increase in coronary vascular resistance, associated with elevated arterial pressure and an absolute reduction in coronary blood flow and coronary sinus oxygen content. The secondary coronary vasoconstriction was also abolished by alpha-adrenergic blockade. Paradoxically, alpha-adrenergic receptor blockade with phentolamine (at constant heart rate and after beta-adrenergic receptor blockade) did not increase coronary blood flow and reduced coronary vascular resistance only slightly. Selective alpha 1-adrenergic receptor blockade with prazosin and trimazosin on different days induced progressively greater reductions in coronary vascular resistance. Trimazosin was the only alpha-adrenergic receptor blocker to elevate coronary blood flow significantly. It is conceivable, but speculative, that withdrawal of alpha-adrenergic tone may involve activation of an intermediate agent, which is a potent coronary vasodilator. Alternatively, withdrawal of alpha-adrenergic tone may be an important mechanism for immediate control of the coronary circulation, but under more chronic conditions it plays a lesser role as a result of suppression by metabolic factors.