Background and Purpose: Superoxide anions can reduce the bioavailability and actions of endothelium-derived NO. In human resistance-sized arteries, endothelium-dependent vasodilatation can be mediated by H2O2 instead of NO. We tested the hypotheses that in resistance arteries from patients with resistant cardiovascular disease (CVD), endothelium-dependent vasodilatation uses mechanisms that are either insensitive to oxidative stress or involve a reactive oxygen species. Experimental Approach: Small arteries were isolated from biopsies of the parietal pericardium of patients undergoing elective cardiothoracic surgery and were studied by immunohistochemical and organ chamber techniques. Key Results: NO-synthases 1, 2 and 3, superoxide dismutase 1 and catalase proteins were observed in the microvascular wall. Relaxing responses to bradykinin were endothelium dependent. During submaximal depolarization-induced contraction, these relaxations were inhibited by inhibitors of NO-synthases (NOS) and soluble guanylyl cyclase (sGC) but not by scavengers of NO or HNO, inhibitors of cyclooxygenases, neuronal NO-synthase, superoxide dismutase or catalase, or by exogenous catalase. During contraction stimulated by endothelin-1, these relaxations were not reduced by any of these interventions except DETCA which caused a small reduction. Conclusion and Implications: In resistance arteries from patients with resistant CVD, endothelium-dependent relaxations seem not to be mediated by NO, HNO or H2O2 although NOS and sGC can be involved. These vasodilator responses proceed during excessive oxidative stress.