Measurements of NO2 in spectra of the rising and setting sun from Mauna Loa, Hawaii, gave a mean of 30 ppt for NOx at 3¿0.5 km altitude in the mid-Pacific troposphere. Natural variations of ~50 percent occurred about this mean together with the systematic nocturnal decay expected from the reaction of NO2 with ozone. The tropospheric column abundance of NO2 exhibited a correlation with the variation of NO2 density at 3 km; the changes in both appear related to the strength of the trade winds and may reflect an overall rising or sinking motion in the lower troposphere. The measured NOx appears to be free of any anthropogenic influence and low enough in density that in the lower mid-Pacific troposphere the source of ozone is downward diffusion from above and not local photochemical production.