In this paper, we study the formation of NO in laminar, nitrogen-dild. methane diffusion flames that are seeded with ammonia in the fuel stream. We have performed numerical simulations with detailed chem. as well as laser-induced fluorescence (LIF) imaging measurements for a range of ammonia injection rates. For comparison with the exptl. data, synthetic LIF images are calcd. based on the numerical data accounting for temp. and fluorescence quenching effects. We demonstrate good agreement between measurements and computations. The LIF corrections inferred from the simulation are then used to calc. abs. NO mole fractions from the measured signal. The NO formation in both doped and undoped flames occurs in the flame sheet. In the undoped flame, four different mechanisms contribute to NO formation. The present calcns. show that the most important pathway is prompt NO, followed by the NNH mechanism, thermal NO, and the N2O mechanism. As the NH3 seeding level increases, fuel-NO becomes the dominant mechanism and N2 shifts from being a net reactant to being a net product. Nitric oxide in the undoped flame as well as in the core region of the doped flames is underpredicted by the model; we attribute this mainly to inaccuracies in the NO recycling chem. on the fuel-rich side of the flame sheet.