In situ measurements of nitric oxide (NO) and nitrogen dioxide (NO2) were made simultaneously from the NASA DC-8 aircraft as part of the Airborne Arctic Stratospheric Expedition. Mixing ratios of NOx (NO+NO2) were typically higher in the Arctic troposphere than in the stratosphere, with median values of 59 and 40 pptv, respectively. In the stratosphere, there tended to be a positive correlation between NOx and water vapor (H2O) and negative correlations between NOx and ozone (O3) and between NOx and total reactive odd-nitrogen (NOy). The ratio of NOx to NOy, in conjunction with NOy, appears to be an excellent tracer of tropospheric or stratospheric air at northern latitudes during winter. Tropospheric NOx was typically 10 to 50% of gas-phase NOy, while in the stratosphere, NOx was typically <10%, and frequently <5% of gas-phase NOy. ¿ American Geophysical Union 1990