The time dependent energy distribution functions (EDF) of N(4S) atoms for altitudes greater than 100 km at 1400 hr local time are calculated by solving the time dependent Boltzmann equation. At time t=0 the time dependent sources and sinks are switched on into a Maxwell-Boltzmann distribution of N(4S) atoms and the time dependence of the EDF is followed by solving the Boltzmann equation. It is found that the non-Maxwellian tail of the EDF reaches a steady-state in 0.01 s giving the fraction of nascent N(4S) atoms that react with O2 before the establishment of steady-state to be less than 1%. The number of NO molecules produced per second, after the establishment of steady-state, by the non-Maxwellian tail of the EDF is at most about 3% of the number of N(4S) atoms in the sources. The last fraction, often referred to as NO reaction efficiency, is about an order of magnitude smaller than that given in recent literature. An important consequence of this study is that the models still calculate less NO in the mesosphere and lower thermosphere than is measured. ¿ 1997 American Geophysical Union