Atomic oxygen affects both the heating and cooling of the mesosphere and lower thermosphere through its recombination and collisional deactivation of CO₂. Recent increases in the accepted value of the deactivation constant, k_o in CO₂‐O collisions enhance the role of the latter process in the dynamics and energy budget of this region. There are strong increases in the radiative cooling due to CO₂ in the 90–120 km region. The sensitivity of the cooling rate in the 15 μm band to variations in atomic oxygen concentration, [O], is enhanced so that the atmosphere in this region is unstable to these variations in concentration. Convection appears possible thereby providing a vertical mixing mechanism which is ineffective in the diffusion of heat but is effective in mixing constituents, and some justification for the use of different eddy diffusivities for heat and constituents. Heating rate calculations are intrinsically more complicated because both the heating and the cooling processes involving atomic oxygen are nonlinear functions of its concentration. The damping of atmospheric waves will be affected because radiative damping will be stronger up to 100 km and the cooling rate dependence on the atomic oxygen mixing ratio affords a new mechanism for photochemical damping. It appears that the overall dynamics and energy budget of this region must be reconsidered.