We describe a step-by-step protocol for measuring the stable products of the nitric oxide (NO) pathway: nitrite, nitrite plus nitrate and nitrate. This described protocol is easy to apply and is about 50 times more sensitive than the commonly used Griess reaction or commercially available assay kits based on the Griess reaction. It also allows the study of minimal changes in the NO pathway. With this method, it takes about 3 h to analyze the above-mentioned stable products in culture supernatants or in various body fluids, and the method has a sensitive linear range of 0.02-10.0 microM. This restricted linear range suggests that the technique is useful for studying small changes of nitrite and nitrate, rather than for routine diagnostic measurements.