A new assay for indirectly detecting nitric oxide in serum was established. Nitrate was reduced to nitrite by activated spongy cadmium with copper ion on a microscale chromatographic column, then the nitrite was measured by the Griess reaction. The cadmium column has good vitality, high speed and low interference for the reducing reaction at pH 9.7. The reducing rates are 96.4%～100.0%. The linear range of the assay is 0～400 μmol·L^-1, the limit is 1.85 μmol·L^-1. The RSDs are 2.56%～3.46%, and the recoveries are 96.4%～102.2%, 95.2%～101.3% and 98.7%～104.4% for NO^-₃, NO^-₂ and NO^-₃/NO^-₂ (95∶5) respectively. This method is easy, rapid and precise, needing only a small amount of sample and reagents, and can be used in clinical assay.