One- and two-dimensional models of thermospheric odd nitrogen are used to explore the consequences of the recently measured fast quenching of N(2D) by O. A large rate coefficient for this reaction profoundly affects the odd-nitrogen chemistry by removing N(2D) as a source of NO and increasing the concentration of N(4S), which destroys NO. The model calculations show that, as the quenching rate increases, the NO and N(2D) densities decrease, while N(4S) densities increase. Comparisons with Atmosphere Explorer and Solar Mesosphere Explorer satellite observations are made. Use of the fast quenching rate in the models causes the NO peak altitude, typically observed near 110 km, to rise to 140 km. The N(2D) densities become 20 times smaller than those observed, while the modeled NO (N(4S)) densities are roughly 2 to 3 times too small (large). Additional measurements of the N(2D)+O quenching rate are clearly warranted. If the quenching rate is indeed very rapid, the chemistry of thermospheric odd nitrogen must be completely reexamined. ¿ American Geophysical Union 1989