In the present study the mechanic activity of SHR and Wistar rat's aorta was evaluated, in vitro, after stimulation by chloride of potassium, phenylephrine, norepinephrine, histamine, serotonin and acetylcholine, before and after the removal of the endothelium. The aorta rings of the rats were taken before the development of the hypertensive state (7th week) and at 18th week (when the SHR rats already showed established hypertension starting since IXth week of life), and successively suspended in a bath for isolated organ. The mechanic activity was measured by isometric transductor. The obtained findings show an increased sensitivity, in SHRs, to (K+), NA and FeE, if these are compared with the control group, since the prehypertensive stage (7th week). The removal of the endothelium didn't modify the response amplitude to K in both the breeds, while the maximum response amplitude, provoked by NA and FeE, significantly increased in SHRs compared to controls. The relaxation induced by the vasodilator agents (Ach-H-5HT) was completely absent in the SHR rats' aorta with established hypertension. In conclusion, these results suggest a functional deterioration of the endothelial cells, in the hypertensive animals, that could contribute to increase the peripheric vascular resistances observed during hypertension.