Measurements of the concentrations of NO and NO2 were made in the rural troposphere during a year's period in 1980--1981, during the summers of 1983, 1984, and 1987, and during the fall of 1984. The field site was located near Niwot Ridge, Colorado, at an elevation of 3 km. NO was measured with a chemiluminescence instrument, and NO2 was photolyzed to NO and measured by the same instrument. The performance of this instrument is discussed in detail. Aspects of the NOx (sum of NO and NO2) concentrations discussed include the overall distribution, the seasonal and diurnal cycles, the interannual variations, and the correlation with meteorological parameters. The meteorological dependence of x> elucidates the mechanisms that transport NOx to the site. Even though the Denver urban area lies downwind from the site with respect to the prevailing winds, it is NOx from that area that predominates and is responsible for the enhanced acidic deposition and elevated ozone levels at the site. The valley-mountain flow that is primarily responsible for this transport also may be an important mechanism for transporting anthropogenic pollution from the planetary boundary layer to the free troposphere. The major photochemical loss processes of NOx are reaction of NO2 with hydroxyl radicals to form HNO3 and the formation of PAN (peroxyacetyl nitrate) and perhaps other organic nitrates in the summer and reaction of NO2 with O3 to form NO3, N2O5, and further oxidized species in the winter. ¿ American Geophysical Union 1990