Objective: To determine if oxygen free radicals derived from xanthine oxidase are involved in the development of salt-induced hypertension. Enhanced production of oxygen free radicals may play a role in hypertension by affecting vascular smooth muscle contraction and provide a mechanism for lesion formation. Methods: Dahl salt-sensitive (Dahl-S) and salt-resistant (Dahl-R) rats were fed either a low-salt, high-salt or high-salt + tungsten diet for 4 wk. In vivo production of superoxide (O2-) was detected by the reduction of a tetranitroblue tetrazolium (TNBT) dye in the rat mesentery, while plasma hydrogen peroxide (H2O2) production levels were determined using a modified electrochemical electrode technique. Results: The tungsten diet lowered the blood pressure of Dahl-S rats compared to high-salt-treated Dahl-S rats, but had no effect on blood pressure in Dahl-R rats. Light absorption of formazan deposits revealed that tungsten-treated Dahl-S rats had reduced TNBT staining along the endothelium of arterioles and venules compared to hypertensive, high-salt-treated Dahl-S rats. In addition, tungsten-treated Dahl-S rats had a lower plasma H2O2 concentration compared to hypertensive, high-salt-treated Dahl-S rats. Conclusions: These findings indicate that xanthinc oxidase-derived oxygen free radicals are involved in the pathogenesis of salt-induced hypertension.