Two recent measurements of the temporal variation of nitric oxide at constant altitude near 40 km are reported. The observations were made at float altitude with a balloon-borne chemiluminescence detector together with in situ ozone measurements. The first measurement was made at 44¿N on September 17, 1987, at an altitude of 40 km from before sunrise until 1000 LT. The second observation was made at the same latitude on June 18, 1988 at 39 km from 0800 to 1230 LT. The precision of the NO measurements is about 10% at these altitudes, obtained through a more accurate determination of the sample flow rate at pressures down to 2 mbar with a reduced ozone/oxygen flow rate. At an altitude of 40 km, nitric oxide was observed to start increasing very rapidly at sunrise when the solar zenith angle reached about 95¿. After the rapid initial buildup, the rate of NO increase stabilized for 3 hours at about 1.2 ppbv/hour. Near 1100 LT at 39 km in summer the NO mixing ratio was observed to become nearly constant. These features of the diurnal variation of NO are in accord with the temporal variation expected from a time-dependent zero-dimensional photochemical model. Mixing ratios of NOy and N2O5 have been determined from the measured temporal variation of NO at 39.5¿0.5 km, using the photochemical model of Pirre et al. (1989). These derived values of the NOy and N2O5 mixing ratios, 15.5¿5.0 ppbv and 1.4¿0.6 ppbv, respectively, are in good agreement with other recent measurements by balloon-borne and space-borne infrared experiments. ¿ American Geophysical Union 1990