Description: Superoxide radical (O2-) is increased in the vessel wall of spontaneously hypertensive rats (SHR) where its blockade potentiates endothelium-dependent vasodilation. The purpose of this study was to determine the role of O2- in the hypertension and renal vasoconstriction of SHR and its interaction with nitric oxide (NO). The antihypertensive action of tempol depends on NO synthesis presumably because O2- inactivates NO and thus diminishes its vasodilatory actions.