Total reactive nitrogen (NOy), nitrous oxide (N2O), methane (CH4), and ozone (O3) were measured on board a balloon launched from Aire sur l'Adour (44 ¿N, 0 ¿W), France on October 12, 1994. Generally, NOy was highly anti-correlated with N2O and CH4 at altitudes between 15 and 32 km. The linear NOy-N2O and NOy-CH4 relationships obtained by the present observations are very similar to those obtained on board ER-2 and DC-8 aircraft previously at altitude below 20 km in the northern hemisphere. They also agree well with the data obtained by the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument at 41 ¿N in November 1994. Slight departures from linear correlations occurred around 29 km, where N2O and CH4 mixing ratios were larger than typical midlatitude values, suggesting horizontal transport of tropical airmasses to northern midlatitudes in a confined altitude region. ¿ American Geophysical Union 1996