Balloon-borne in situ measurements of total reactive nitrogen (NOy) and nitrous oxide (N2O) were made from Kiruna (68 ¿N, 21 ¿E), Sweden on February 11, 1995. Ten hours later, N2O was again measured by an infrared spectrometer flown on another balloon launched from Kiruna. Both observations were made inside the polar vortex between 380 K (~14 km) and at least 675 K (~26 km). In the winter of 1994--1995, temperatures at 475 K (~19 km) inside the vortex were extremely low, sometimes lower than ice frost point, especially from mid-December to mid-January. The NOy profiles obtained during both the ascent and descent revealed layered structures between 15 and 20 km with mixing ratios ranging from 2.7 to 9.3 parts per billion by volume (ppbv). The observed N2O profiles indicate significant downward transport of air due to diabatic cooling in the winter. To quantify the degree of irreversible removal of NOy (denitrification) between 12 and 28 km, the unperturbed values of NOy (i.e., NOy*) were estimated from the observed N2O values using the NOy-N2O relationship obtained at midlatitudes by the atmospheric trace molecule spectroscopy ATLAS mission and in situ aircraft and balloon-borne measurements. The largest denitrification was observed at 19¿0.5 km, where the NOy values were lower than the NOy* values by ~10 ppbv, corresponding to a 70% removal of NOy. In spite of the large uncertainty in NOy* the NOy values generally agreed well with the NOy* values at ~14 km as wellas between 23 and 28 km. The relationship between NOy and N2O measured between 23 and 28 km agreed with that measured above 40 km at northern midlatitudes in fall, indicating that the air masses sampled at 23--28 km over Kiruna were transported from the midlatitude upper stratosphere followed by the descent inside the vortex. ¿ 1998 American Geophysical Union