A simple and sensitive method is presented to measure the unstable molecule nitric oxide (NO) by reconversion of nitrate/nitrite to NO. Nitrate and nitrite are the stable degradation products of NO that accumulate in supernatants of biological samples that release nitric oxide. First, nitrate is enzymatically converted to nitrite using nitrate reductase. In a second step, nitrite is reduced to equimolar NO concentrations by an acidic iodide solution and quantified with an amperometric Clark-type electrode. This method provides the ability to assess basal-and agonist-stimulated cumulative NO formation in different biological models and is a sensitive alternative to the widely used Griess assay.