A subset of Pacific Exploratory Mission in the Western Pacific Ocean (PEM-West B) data (northwestern Pacific, March 1994), selected to represent upper troposphere (UT) midlatitude conditions, is analyzed to answer the following questions: (1) Is there a shortfall in total reactive nitrogen (NOy) as measured during PEM-West B in this region? (2) If so, what are the likely contributions of interfering constituents like HCN or of other reactive nitrogen species? For question 1 our analyses show that 87% of total reactive nitrogen measured in the UT is accounted for by NOx+HNO3+PAN (similar to Kondo et al. , Talbot et al. , and Singh et al. <1997a, b>). For question 2 we find that less than 20 pptv (<5% of mean NOy) is possibly HCN. A one-dimensional model that simulates mean mixing ratios of this PEM-West B data is used with a Monte Carlo approach to explore other candidates for unmeasured nitrogen species and NOy partitioning. Using standard gas-phase reactions with varying rate coefficients <Thompson and Stewart, 1991; Stewart and Thompson, 1996>, it is found, on average, that NOx+HNO3+PAN=399 pptv (observed mean equal to 432¿97 pptv). A more complete inventory for total reactive nitrogen (&Sgr;NOi=NOx+HNO3+PAN+HNO4+CH3O2NO2+alkyl nitrates+C2H5O2NO2) is 494¿91 pptv, with 19% consisting of HNO4+CH3O2NO2+alkyl nitrates. Thus, whether unmeasured forms of reactive nitrogen are present or not, total reactive nitrogen as measured at midlatitude UT during PEM-West B is accounted for within the measurement uncertainty. The greatest kinetics uncertainties are in thermolytic losses for HNO4, PAN, and CH3O2NO2 (120--150% by the method of Stewart and Thompson <1996>). Nonetheless, comparison with PEM-West B data shows that panel-recommended kinetics expressions <Demore et al., 1994> can explain reactive nitrogen observations without invoking extreme rates or heterogeneous processes. In summary, large concentrations of unmeasured reactive nitrogen species were not prevalent during midlatitude UT PEM-West B sampling although the observed shortfall (13%) can be explained by HNO4+alkyl nitrates+CH3O2NO2. Agreement between theory and observations may also reflect improved instrument capabilities for measuring reactive nitrogen. ¿ 1997 American Geophysical Union