As part of NASA's Arctic Boundary Layer Expedition 3A and 3B field measurement programs, measurements of NOx, HNO3, PAN, PPN, and NOy were made in the middle to lower troposphere over Alaska and Canada during the summers of 1988 and 1990. These measurements are used to assess the degree of closure within the reactive odd nitrogen (NxOy) budget through the comparison of the values of NOy measured with a catalytic convertor to the sum of individually measured NOy(i) compounds (i.e., &Sgr;NOy(i)=NOx+HNO3+PAN+PPN). Significant differences were observed between the various study regions. In the lower 6 km of the troposphere over Alaska and the Hudson Bay lowlands of Canada a significant fraction of NOy budget (30 to 60%) could not be accounted for by the measured GSNOy(i). This deficit in the NOy budget is about 100 to 200 parts per trillion by volume (pptv) in the lower troposphere (0.15 to 3 km) and about 200 to 400 pptv in the middle free troposphere (3 to 6.2 km). Conversely, the NOy budget in the northern Labrador and Quebec regions of Canada is almost totally accounted for within the combined measurement uncertainties of NOy and the various NOy(i) compounds. A substantial portion of the NOy budget's ''missing compounds'' appears to be coupled to the photochemical and/or dynamical parameters influencing the tropospheric oxidative potential over these regions. A combination of factors are suggested as the causes for the variability observed in the NOy budget. In addition, the apparent stability of compounds represented by the NOy budget deficit in the lower-altitude range questions the ability of these compounds to participate as reversible reservoirs for ''active'' odd nitrogen and suggest that some portion of the NOy budget may consist of relatively unreactive nitrogen-containing compounds.