On September 19--20 and October 4, 1985, in situ measurements of nitric oxide and ozone were carried out on board balloons launched from Aire sur l'Adour (44 ¿N, 0 ¿W) in France as a part of the Middle Atmosphere Program Global Budget of Stratospheric Trace Constituents NOx campaign. On September 19 a daytime NO profile from 6 to 33 km and the variation of NO in the afternoon at a float altitude of about 32 km were measured precisely. In the afternoon, when the solar zenith angle was between 48¿ and 70¿, NO concentration was fairly stable, probably to within 10%. The slow decrease of NO near sunset at solar zenith angles larger than 80¿ was observed. A calculation assuming photochemical equilibrium between NO and NO2 has shown that the decay of NO toward sunset can be explained mainly by the decrease of the photodissociation rate at NO2 at large solar zenith angles. On October 4 the variation of NO at an altitude of about 33 km was measured for 2.5 hours after sunrise. Following the rapid buildup, which takes place for 30 min after visible sunrise, NO increases at a rate of approximately 1.2 parts per billion by volume (ppbv) per hour. From the rate of increase in NO in the morning, the N2O5 mixing ratio at sunrise at 33 km has been estimated to be 3.9¿1.5 ppbv. ¿ American Geophysical Union 1988