Simultaneous in situ measurements of NO, NO2, and O3 were obtained from balloon flights near 50¿ N in August and December 1982. NOx(NO+NO2) was reduced by a factor of 10 in the 21- to 28-km region in winter. Both NO and NO2 exhibited very sharp vertical gradients in the vicinity of 29 km. NO, for example, increased by nearly 20 times over only a 1-km increase in altitude. By considering air mass trajectories for the time of the winter flight, the reduction in NOx in the lower and middle stratosphere was determined to be consistent with photochemical conversion, principally to N2O5. ¿ American Geophysical Union 1987