The paper reports results of a study addressing the importance of the different chemical pathways responsible for nitrogen oxides (NOx) formation in lean-premixed combustion, and especially the role of the nitrous oxide pathway relative to the traditional Zeldovich pathway. The primary zone, of uniform fuel/air ratio, is modeled as a micromixed well-stirred reactor, representing the flame zone, followed by a series of plug flow reactors, representing the post-flame zone. The fuel is methane. Verification of the model is based on the comparison of its NOx output to experimental results published for atmospheric pressure jet-stirred reactors and for a 10 atm porous-plate burner. Good agreement between the modeled results and the measurements is obtained for most of the jet-stirred reactor operating range.