High-latitude observations of fluorescent nitric oxide gamma bands were made before and during a strong magnetic storm with the Ogo 4 ultraviolet spectrometer. Brightness measurements of the (1--0) gamma band of nitric oxide indicate a slow buildup of NO during the disturbed period. The NO column density reaches a value as high as a factor of 8 greater than the mid-latitude value and shows no correlation with the brightness of the instantaneous aurora. A time-dependent model caclulation indicates that the ionization and dissociation of N2 by auroral electrons can increase the NO and N(4S) densities. This increase is dependent on the intensity and duration of the auroral precipitation and on the branching ratio of N(2D) production by dissociation of N2. A steady state is not reached for NO until 105 s in an aurora characterized by an energy flux of 10 ergs/cm2s. In that case the NO equilibrium density at 110 km ranges between 3¿108 and 6¿109 cm-3, depending on the model adopted. Dissociation by the solar ultraviolet radiation competes with horizontal and vertical transport as a loss process for the nitric oxide produced by the aurora. A high NO+/O2+ ratio is to be expected in te period following a strong auroral precipitation.