The transmural distribution of alpha-receptor coronary vasoconstriction was studied in 14 closed-chest, morphine- and chloralose-anesthesized dogs. The alpha-receptor-blocking agent, phenoxybenzamine (0.25 mg/kg), was infused into either the anterior or circumflex coronary artery while collection of coronary sinus blood minimized recirculation. The left main coronary artery was cannulated and perfused at constant pressure. Myocardial lactate extraction and blood flow (9 micron radioactive microspheres) were measured during adrenergic activation with intracoronary norepinephrine (2 micrograms/min) at coronary artery pressures of 100, 70, 50, and 38 mm Hg. Flow was 15-25% less in the control region (alpha-receptors intact) than in the alpha-receptor-blocked region, in all layers of the left ventricular wall, at coronary pressures of 100 and 70 mm Hg. When coronary pressure was 50 mm Hg, lactate production resulted (-4% extraction) and significant alpha-receptor vasoconstriction was observed in the outer layer, but was marginal (P = 0.043) in the inner layer, of the left ventricle. Flows were not different in control and alpha-receptor-blocked regions at a coronary pressure of 38 mm Hg (lactate extraction -49%). These data indicate a uniform transmural alpha-receptor vasoconstriction at normal coronary artery pressures that diminished as the heart was progressively underperfused.