A simple formulation of the nitric oxide cooling by the emission of the 5.3 &mgr;m fundamental band in the terrestrial thermosphere indicates that this process is an important mechanism which has been omitted previously in all thermospheric models. The cooling effect is highly variable as a consequence of its strong temperature dependence and of the observed variability of the nitric oxide concentration. The infrared emission is never in local thermodynamic equilibrium above 120 km altitude and the predominant cooling probably occurs during No auroral enhancements.