We studied the effects of sevoflurane and isoflurane on coronary hemodynamics in relation to myocardial oxygen supply and demand. Dogs were anesthetized with pentobarbital and fentanyl and received isoflurane or sevoflurane. An electromagnetic flow probe and a pair of piezoelectric crystals were placed on the left circumflex coronary artery (CX) to measure CX flow and diameter. The dogs were randomly assigned to receive isoflurane or sevoflurane at a dose of 0.75 and 1.5 minimum alveolar anesthetic concentration (MAC). The CX diameter decreased in parallel with the decrease in mean arterial pressure during both anesthetics. The CX blood flow decreased in parallel with the decrease in myocardial oxygen consumption (MVo2) during sevoflurane, whereas it did not change in spite of the decrease in MVO2 during isoflurane. The CX vascular resistance decreased significantly during isoflurane but not during sevoflurane. Moreover, the myocardial oxygen extraction ratio (Mo2exr) decreased at 0.75 and 1.5 MAC isoflurane and at 1.5 MAC sevoflurane, and the decrease in Mo2exr was significantly greater during isoflurane than during sevoflurane. The results suggest that sevoflurane is a less potent coronary arteriolar dilator than isoflurane, and that neither sevoflurane or isoflurane has a direct effect on the diameter of large coronary arteries.