Measurements of nitric oxide (NO) and ozone (O3) in the free troposphere were made on board aircraft over the western Pacific Ocean during the period between March 7 and 10, 1989. The aircraft flew out from Narita (35¿N) in Japan to Biak (1¿S) in Indonesia along the longitude of 140¿¿5¿E. The return flights from Biak were made at longitudes closer to the Asian continent ranging between 120¿ and 135¿E. Both NO and O3 mixing ratios were observed to have pronounced latitudinal dependence. South of 6¿N, very low NO and O3, with mixing ratios of about 7 parts per trillion by volume (pptv) and 14 parts per billion by volume (ppbv) respectively, were observed to prevail at a 4.5 km altitude. Near the latitude of 10¿¿3¿N, NO and O3 showed a sharp increase toward the north, coinciding with a sharp fall in the dew point. North of 17¿N, the median NO and ozone values in the continental airmass were 19 pptv and 50 ppbv, respectively, while corresponding values in maritime air mass were 21 pptv and 30 ppbv. These NO mixing ratios north of 17¿N over the western Pacific Ocean are somewhat larger than the values obtained over the northeastern and north central Pacific Ocean probably due to a stronger influence of the outflow from east Asia on the western Pacific region. A pronounced peak in NO mixing ratio which reached 40 pptv was observed near 20¿N which indicates more direct influences of anthropogenic NOx emissions from Asia. ¿ American Geophysical Union 1993