The origin of NOx in the upper troposphere over the central United States is examined using aircraft observations obtained during the SUCCESS campaign in April-May of 1996. Correlations between NOy (sum of NOx and its oxidation products) and CO at 8--12 km altitude indicate that NOx originates primarily from convective transport of polluted boundary layer air. Lightning and aircraft emissions appear to be only minor sources of NOx. Chemical steady state model calculations constrained by local observations of NO underestimate the measured NOx/NOy concentration ratio at 8--12 km altitude by a factor of two on average. The magnitude of the underestimate is correlated with concentrations of condensation nuclei, which we take as a proxy for the age of air in the upper troposphere. We conclude that the NOx/NOy ratio is maintained above chemical steady state by frequent convective injections of fresh NOx from the polluted boundary layer and by the long lifetime of NOx in the upper troposphere (5--10 days). In contrast to previous studies, we find no evidence for fast heterogeneous recycling from HNO3 to NOx in the upper troposphere. ¿ 1998 American Geophysical Union