A chemiluminescent balloon-borne sonde, designed to measure nitric oxide with a high degree of precision, has been developed. Within the framework of the MAP/GLOBUS campaign, the sonde was launched on September 20, 1983, from Aire sur l'Adour, France (44 ¿N, O ¿W). The balloon was piloted to perform an excursion from 32 to 22 km during the flight and returned to float at 32 km 1 hour before sunset to enable a sunset study to be made. The first ascent and descent profiles, obtained with solar zenith angles of less than 57¿ coincided even in detail to within 5% between 22 and 32 km, which not only assured instrumental precision but the unchanging NO mixing ratio during the day. The first ascent profile showed a rapid increase in the NO mixing ratio from 50 pptv at 7 km to 250 pptv at the tropopause indicating a downward transport of NO into the troposphere. After local sunset, the NO concentration was observed to rapidly decrease to 0¿10 pptv.