A chemiluminescent NO detector was launched from Uchinoura, Japan (31 ¿N), at 0030 JST on July 29, 1987. The balloon reached a float altitude of 26 km at around 0200 JST and stayed at this altitude until 1600 JST when it started a slow descent. Near local sunrise, when the solar zenith angle reached 92.7¿, the NO concentration started to increase rapidly. This rapid increase continued for about 30 minutes. The NO concentration continued to increase, albeit more slowly, for a further 7 to 8 hours, and then became stable. The NO diurnal variation is calculated by a time-dependent photochemical model assuming a 14 ppbv total odd nitrogen concentration, constrained by ozone and temperature measured simultaneously with NO; the result agrees quite well with the observed temporal NO variation. The observed slow increase in NO can quantitatively be explained by the photodissociation of N2O5. The N2O5 concentration just before sunset is estimated to be 1.9¿0.4 ppbv at 26 km. ¿ American Geophysical Union 1989