Laser-induced fluorescence (LIF) measurements of NO concentration were obtained in a series of flat, laminar, premixed counterflow CH4/O-2/N-2, flames at atmospheric pressure with equivalence ratios ranging from 0.65 to 1.5. The counterflow configuration permits measurement of NO concentration profiles in the preheat zone, through the flamefront and into the postflame region of premixed flames. NO predictions using the Sandia opposed diffusion flame code in conjunction with the GRI reaction mechanism (version 2.11) were compared to the measured NO concentrations. Under lean conditions, remarkable agreement was found between predictions and measurements; however, this agreement broke dawn in moderately rich flames both with respect to the peak NO concentrations and to the relative shape of the NO profiles. Pathway, quantitative reaction path, and sensitivity analyses are used to further analyze this behavior and to suggest key reaction rate coefficients that could be responsible for the observed results. (C) 2001 by The Combustion Institute.