Probe sampling measurements of the concentrations of nitric oxide in the post-flame zone of methane + oxygen + nitrogen flames doped with ammonia (0.5% of the fuel) are reported. The goal of this work was to analyze formation of NOx from fuel-N under well-controlled conditions. A Heat Flux method was used for stabilization of non-stretched flames on a perforated plate burner at atmospheric pressure. Dilution ratios of oxygen, O-2/(O-2 + N-2), were varied from 0.16 to 0.209. The concentrations of O-2, CO, CO2 and NOx were measured by means of a non-cooled quartz probe at different axial distances from the burner. Measured burning velocities for these flames and concentrations of the major species (O-2, CO, CO2) agree well with those of the flames of methane + oxygen + nitrogen within an experimental accuracy. The concentrations of NOx in the post-flame zone have a maximum near the stoichiometry. These measurements were compared to predictions of detailed kinetic models and to similar experiments in flames of methane + oxygen + carbon dioxide doped with ammonia. In (CH4 + NH3) + O-2 + CO2 mixtures the modeling over-predicts the measured concentrations of NOx however the experimental trends are well reproduced. In (CH4 + NH3) + O-2 + N-2 mixtures the plots of the concentrations of NOx in the post-flame zone as a function of the stoichiometric ratio differ qualitatively from that in (CH4 + NH3) + O-2 + CO2 mixtures. The modeling is in satisfactory agreement with the experiments in lean flames, while in rich flames it is not.