The effects of 2 dissimilar anesthetic regimens on hemodynamic factors affecting the risk of myocardial ischemia were compared. Myocardial blood flow was recorded as an indicator of O2 supply during autoregulation and maximal vasodilatation at various coronary arterial perfusion pressures while myocardial O2 demand was stable; the relationship between pressure and blood flow was used as an index of coronary vascular reserve. Pressure-flow relations in the left circumflex coronary artery during light halothane-O2-relaxant anesthesia and during NO-O2 relaxant anesthesia were compared in 11 dogs. Changes in coronary arterial pressure were induced by hydraulic constriction and recorded through a small catheter in the circumflex coronary artery for each anesthetic regimen during autoregulated and during maximally vasodilated coronary arterial blood flows. Flow was measured by an electromagnetic flow transducer on the circumflex coronary artery and by radioactive microspheres. There were 2 major differences between the 2 anesthetic regimens. Myocardial O2 demand was less during halothane anesthesia, as measured by myocardial O2 consumption, wall tension or the rate-pressure product. Myocardial blood supply decreased similar during halothane anesthesia. With halothane anesthesia the pressure-flow relationship during maximal vasodilation was significantly shifted to the left. This parallel shift to the left reflects the lower coronary arterial perfusion pressure at which flow becomes zero, and is best explained by the vascular waterfall theory. Minimal coronary vascular resistances are similar with the 2 anesthetic regimens since the maximal vasodilatation lines are parallel, but diastolic intramyocardial tissue pressure is probably lower with halothane, accounting for the lower coronary arterial diastolic pressure at which flow stops. This shift reflects the lower coronary arterial perfusion pressure at which subendocardial ischemia occurs when autoregulation of blood flow is present. Dogs lightly anesthetized with halothane evidently have greater coronary vascular reserves than do dogs anesthetized with NO.