Measurements of NO and NOy were carried out during NASA's Pacific Exploratory Mission-West A. In total, 18 aircraft flights were made over the Pacific Ocean, predominantly over the western Pacific Ocean in September and October 1991. NO and NOy were measured using a chemiluminescence instrument, and NOx was calculated from NO using a chemical box model. The measurements were carried out from 0.3 to 12 km in altitude. The NO, calculated NOx ((NOx)mc), and NOy mixing ratios in continental air were significantly higher than in maritime air. In maritime air, NO increased with altitude. The median values of NO in the boundary layer and the lower, middle, and upper troposphere were 3.7, 5.1, 11.5, and 26.6 parts per trillion by volume (pptv), respectively. In continental air, NO and (NOx)mc mixing ratios revealed a C-shaped profile. The median NO values observed in the four altitude regions were 37.8, 17.5, 18.2, and 53.2 pptv, respectively. NOy did not show any apparent altitude dependence either in maritime or in continental air. In maritime air, median NOy values in the lower, middle, and upper troposphere ranged between 211 and 226 pptv and in continental air between 382 and 401 pptv. The lowest values of NOy, PAN, and O3 were observed in tropical air masses throughout the entire altitude region. In the middle and upper troposphere of the high-latitude air masses, NO and (NOx)mc values were the lowest, although NOy mixing ratios were similar to those in continental air masses. PAN, O3, CO, CH4, and C2H6 data were used to study the budget of reactive nitrogen over the Pacific Ocean. O3 mixing ratios were found to be correlated with those of (NOx)mc, NOy, PAN, and CH4, although the degree of correlation varied with air mass and altitude. These correlations, together with the profiles of these species, suggest that photochemical production of O3 from precursor species over the continent is important for the O3 budget in the troposphere over the western Pacific Ocean. ¿ American Geophysical Union 1996