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The ultimate goal of radiotherapy is to induce irreversible damages in genetically unstable, fast‐growing cancer cells while minimizing the cytotoxic effects on host tissues. The satus of the tumor vasculature is particular because it is located within the tumor but mostly arises from host cells. The aim of this study was to characterize the effects of low‐dose irradiation on the function of endothelial cells lining tumor vessels. Using isolated arterioles mounted on a pressure myograph, we first documented that the nitric oxide (NO)‐mediated vasorelaxation that was defective in tumor vessels was completely restored following local tumor irradiation. Immunoblot analyses revealed that this was attributable to an increase in the abundance of the endothelial NO synthase while the expression of its physiological inhibitor, caveolin‐1, was reduced. We further showed that the potentiation of the NO‐dependent pathway induced a …