The mechanical reactivity of SHR and Wistar rats' aortas was evaluated, in vitro, after stimulation by potassium chloride (K+), phenylephrine (Phe), norepinephrine (NE), histamine (H), serotonin (5-HT) and acetylcholine (Ach), before and after the removal of the endothelium. The aortic rings were taken at different ages and stages of the hypertensive state (that in the SHRs started at the ninth week of life) and successively suspended in a bath for isolated organs. The mechanical activity was measured by isometric transducers. In SHR rats an increased sensitivity to K+, NE and Phe compared with the control group, and preceding the hypertensive stage, was found. The removal of the endothelium did not modify the response amplitude to K in both the breeds, while the maximum response amplitude, provoked by NE and Phe, significantly increased in SHRs compared to controls. The relaxation induced by vasodilator agents (Ach, H, 5-HT) was significantly reduced in the SHR rats' aortas with initial hypertension (12th week) if compared with Wistar rats at the same age. The release response was completely absent in the SHR rats' aortas with established hypertension (18th week). In conclusion, these results suggest that, in hypertensive rats, a functional deterioration of the endothelial cells occurs: this may contribute to the increase in peripheral vascular resistance observed during hypertension.