Simultaneous measurements of O3, NO, CO, H2O vapor, and temperature over the eastern and central North Pacific Ocean are analyzed to infer the role of photochemistry in the budget of tropospheric ozone. A statistical analysis of the data reveals a close correlation between ozone and nitric oxide levels and an anticorrelation between these two constituents and water vapor, thus implying the existence of a major ozone source associated with NO-rich air parcels from the upper troposphere and/or lower stratosphere. The NO in these parcels is believed to have originated from the stratosphere, from air detrained from active thunderclouds, or from some combination thereof. The statistical analysis also indicates the existence of an additional source of secondary importance associated with CO-rich air parcels, possibly of continental origin. Photochemical simulations of ozone photochemistry for the conditions encountered during the flights indicates, on average, a net photochemical source of ozone in the free troposphere and a net sink in the boundary layer. The free tropospheric, photochemical source of ozone is found to be strongly correlated with NO, implying that regions of enhanced NO are also regions of enhanced ozone production. A similar statistical correlation between ozone photochemical production and CO levels was not found. It is estimated that on average, during the period of the fall operation, photochemistry provided a net source of ozone to the free troposphere overlying the eastern and central North Pacific Ocean of about 5¿1010 molecules cm-2 s-1 and a net sink of ozone to the boundary layer overlying this region of about 3¿1010 molecules cm-2 s-1. ¿American Geophysical Union 1987