The vascular actions of local anesthetics are important in determining the uptake and distribution of these agents from their site of injection as well as influencing their hemodynamic effects once absorbed. Because of the importance of the endothelium in determining of modulating the vascular response of a wide variety of agents, cumulative dose-dependent vasular effects of lidocaine, mepivacaine and bupivacaine on isolated rings of thoracic aorta in normotensive rats(NTR) and spontaneously hypertensive rats(SHR) were studied in the presence and absence of intact endothelium. The results were as follows ; The body weight of NTR and SHR averaged 274.71±55.80(N = 38) and 241.43±17.73gm(N = 18) and mean arterial pressure was 74.4l±3.60 and 129.34±2.89mmHg respectively. The mean absolute value of the contraction induced by 5Xl0(-6) M phenylephrine was 3.27±0.98(N = 18) and 2.3l±50.64gm(N = 18) with intact endothelium and 3.12±0.92 and 2.46±0.87 gm without intact endothelium in aortic rings of NTR and SHR respectively. In the response to local anesthetics in preparation with resting tension(1.0 gm), lidocaine and mepivacaine in concentration of 10(-3) to 1.25X10(-2) M not produced dose dependent contraction in aortic ring with intact endothelium from NTR. but bupivacaine produced dose-dependent contraction in aortic rings with intact endothelium from NTR. In the aortic rings from NTR and SHR previously contracted with phenylephrine, lidocaine in contraction of 10(-3) to 1.25X10(-2) M caused dose related relaxation in aortic rings with or without endothelium but in concentration of 10(-3) to 510(-3) M, aortic rings with endothelium were more relaxed than those af without endothelium in NTR. In SHR, aortic rings without endothelium in concentration of 5X10(-3) to 1.25X10 M were more significantly relaxed than those of with endothelium. In aortic rings from NTR previously contracted with phenylephrine, mepivacaine caused dose-related relaxation, which was more profound in SHR. In aortic rings with endothelium from NTR previously contracted with phenylephrine, bupivacaine in concentration of 10(-3) to 1.5X10(-3) M caused a relaxation and in concentration of 2.5X10(-3) to 7.5X10(-3) M and 1.25X10(-3) M caused a relaxation again. But in the aortic rings without intact endothelium, bupivacaine caused dose-related relaxation. In the aortic rings without intact endothelium, bupivacaine caused dose-related relaxation in NTR. In the aortic rings from SHR previously contracted with phenylephrine, bupivacaine caused dose-related relaxation, which was more profound than those of NTR. The local anesthetics appear to exert their relaxant effect on endothelium independently and more profoundly in SHR.