The relaxing effects of L-arginine and acetylcholine were compared on contracted thoracic aortic rings from Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) in the present study. Results demonstrated that both vasodilatations induced by L-arginine and acetylcholine in WKY were greater than those in SHR. The maximal relaxation induced by acetylcholine was greater than that induced by L-arginine in both WKY and SHR. However, the difference of relaxation induced by L-arginine was less than that induced by acetylcholine between WKY and SHR. In addition, phenylephrine caused contractions, which were similarly augmented after the removal of the endothelium or the treatment with N omega-nitro-L-arginine in aortic rings from WKY and SHR. On the aorta preparations from SHR a larger contraction was produced by phenylephrine and the maximal relaxations produced by L-arginine and acetylcholine were significantly reduced. These effects were not seen after the endothelium had been removed. We suggest that the ability of the endothelium to synthesize NO may be reduced in hypertension.