Experimental facilities used in studying the performance of probes and sampling systems for measuring NO are described. A critical review of the literature on probe measurements of NOx is given with emphasis on reported results indicating that probes may perturb the total concentration of NOx in a flame. Also, sample line and chemiluminescent analyzer phenomena are reviewed. A model of probe aerodynamics including heat transfer is presented. Kinetics of NO loss are examined and quenching criteria for measuring nitric oxide in flames are given. Sampling probes are described that were designed to preserve NO and are suitable for measurements on small and large combustors. Probes were designed to cool the gases both convectively and aerodynamically. Performance of these probes is compared with model predictions. Concentrations of nitric oxide were measured using several probes for each of three flame environments. The values measured with each probe are compared and related to seed levels of NO. In addition, concentration profiles required to compare probe measurements with optical measurements are provided.