A method was developed for the on-line analysis of nitrogen stable isotopes at the natural abundance level in NO in order to study the NO contribution to the nitrogen cycle in ecosystems and in the atmosphere. The method enables a quick and accurate determination of 15N/14N ratios for NO and consists of the following steps: (a) accumulation of NO from air samples on a molecular sieve of 5 A, (b) desorption of NO from the molecular sieve during 15 min of heating at 350 degrees C (an offset of Deltadelta 4.6% must be corrected for), (c) trapping and cryofocusation of the desorbed NO on a PoraPlot Q matrix at -196 degrees C during heating, (d) release of the trapped NO from the PoraPlot Q matrix followed by chromatographic separation, reduction to N2, and isotopic composition analysis. A minimum sample size of 125 nmol of NO is recommended. A correction function for the calculation of the delta15N-NO values was introduced for sample sizes from 125 to 220 nmol of NO. Measurements of NO in automobile exhaust have proven the applicability of the developed method.