Halothane, enflurane, and isoflurane are generally regarded as vasodilators. This property has been attributed to a direct action on vascular smooth muscle or the inhibition of vasoconstricition by endogenous neurohumoral substances. Because of the importance of the endothelium in determining of modulating the vascular responses of a wide vareity of agents, vascular effects of halothane, enflurane and isoflaurane on isolated rings of thoracic aorta in Sprague-Dawley rats were studied in the presence and absence of intact endothelium. Halothane, enflurane and isoflurane induced relaxation on thoraeic aortic rings precan-tracted with 50mM KCl both with and without endothelium. Halothane also induced vasodilation in both aortic rings precontracted with 10-6 M phenylephrine. And enflurane and isoflurane induced vasodilation in denuded aortic rings precontracted with phenylephrine. But endothelium intact rings demonstrsted significant(p<0.05) vasoconstriction at low concentrations of both enflurane and isoflurane followed by vasodilation at higher concentra- tion precontracted with phenyephrine. These results suggest that at low concentration and intact rings, enflurane and isoflurane eause vasoconstriction through inhibition of basal EDRF production and /or stimulation of the release of an endothelium derived constricting factor. At higher concentration, a direct vasodilating effect of the anesthetic predominance.