Simultaneous in situ measurements of NO, NOy, HNO3, CO, CO2, O3, and aerosols were performed in the midlatitude upper troposphere (UT) and lower stratosphere during the Stratosphere-Troposphere Experiment by Aircraft Measurements (STREAM) 1998 summer campaign. The campaign focused on the region around Timmins in the Canadian province of Ontario (79.3 ¿W, 48.2 ¿N), close to the polar jet stream that rapidly transports trace species across the Atlantic Ocean. This paper focuses on the origin of total reactive nitrogen (NOy) in the UT, as our measurements show strong variations, which reflect large local sources. In situ production by lightning, stratospheric downdraft, aircraft emissions, and upward transport of polluted boundary layer air are discussed in two case studies as potential contributors. We use correlations among NO, NOy and CO to distinguish between transport from the boundary layer and in situ formations. Lightning production of NOx is found to be a strong contributor to the budget of NOy during high NOy episodes. ¿ 2001 American Geophysical Union