A detailed N/H reaction mechanism has been developed and validated in comparison with experimental data for ammonia pyrolysis in shock waves (Davidson et al., Int. J. Chem. Kinet, 1990, 22:513). It has been shown that incorporation of the reactions with N2H3 and N2H4 into the mechanism significantly influences calculated rise-time and peak concentrations of the NH and NH2 radicals if the currently adopted rate constant of the reaction NH3 + NH2 = N2H3 + H-2 (22) is employed. A sensitivity analysis reveals which reactions are critical for the quality of the modeling in particular experimental conditions. The choice of the rate constants for these reactions is discussed. It has been found that only significant decrease of the reaction (22) rate constant can improve the agreement between the modeling and experimental data. The best fit in the range 2200 - 2800 K is met with the rate constant k(22) = 1.0E+11 T-0.5 exp(-21600/RT).