Absolute two-dimensional concentration distributions of nitric oxide have been measured in a fully transparent square piston engine with planar laser induced fluorescence. A calibration procedure was used to obtain absolute NO concentrations. The NO formation was studied in the propane/air fueled engine as a function of air/fuel ratio, crank angle and load. Simultaneous measurements of NO distributions by LIF and temperature fields via Rayleigh scattering allowed to study the correlation of NO formation and temperature. It was found that the measured NO was mainly formed via the Zeldovich mechanism.