The process Na + O2 + M → NaO2 + M apparently is the first in a sequence of reactions which chemically controls the sodium content of the mesosphere. The effectiveness of this reaction is reduced by NaO2 + O → NaO + O2, which generates only one tenth as much NaO as does the Chapman process Na + O3 → NaO + O2, using current rate coefficients. Hence Chapman's mechanism, which requires the formation of some Na(2P) when NaO reacts with O, remains the principle source of the sodium D-lines at night. However, the net loss of sodium by Na + O3 → NaO + O2 is small since the reaction between NaO and O is fast and since >>3> in the mesosphere. The chemistry of the nightglow, a catalystic cycle with sodium as the catalyt, is shown to be virtually uncoupled from that of the sodium content. It follows that seasonal variations of the content and the nightglow can be fairly independent of each other, as has been observed. Observations that both mesospheric ozone and the sodium nightflow attain maxima during the equinoxes is in qualitative accord with the Chapman mechanism as the dominant source of the sodium nightglow at 5893 ¿.