In catalysing the reversible hydration of CO₂ to bicarbonate and protons, the ubiquitous enzyme carbonic anhydrase (CA) plays a crucial role in CO₂ transport, acid-base balance and in linking local acidosis to O₂-unloading from hemoglobin. Considering the structural similarity between bicarbonate and nitrite, we hypothesized that CA utilizes nitrite as a substrate to produce the potent vasodilator nitric oxide (NO) to increase local blood flow to metabolically active tissues. Here we show that CA readily reacts with nitrite to generate NO particularly at low pH and that the NO produced in the reaction induces vasodilation in aortic rings. This reaction occurs under normoxic and hypoxic conditions and in various tissues at physiological levels of CA and nitrite. Furthermore, two specific inhibitors of the CO₂ hydration, dorzolamide and acetazolamide, increase the CA-catalysed production of vasoactive NO from nitrite. This enhancing effect may explain the known vasodilating effects of these drugs and indicates that CO₂ and nitrite bind differently to the enzyme active site. Kinetic analyses show a higher reaction rate at high pH, suggesting that anionic nitrite participates more effectively in catalysis. Taken together, our results reveal a novel nitrous anhydrase enzymatic activity of CA that would function to link in vivo main end-products of energy metabolism (CO₂/H⁺) to generation of vasoactive NO. The CA-mediated NO production may be important to the correlation between blood flow and metabolic activity in tissues, as occurring for instance in active areas of the brain.