The LIMS experiment sounded the upper atymosphere from late October 1978 to late May 1979 and provided vertical profiles of atmospheric temperature, O3, H2O, HNO3, and NO2. The data were collected over the latitude range from 64¿S to 84¿N, both night and day. We have used the technique of radiance averaging before retrieval to measure the altitude distribution of NO2 over the altitude range from the lower stratosphere into the mesosphere. Observations in the polar winter night region northward of about 70¿N reveal NO2 levels near 175 ppbv at about 70 km, and they show a significant longitudinal variability (factor of 4 to 7). A definite temporal trend exists, showing a buildup of mesospheric and stratospheric NO2 during the polar night and a subsequent slowing of the increase or decline after sunlight returns, depending on altitude. These results provide further insight regarding the importance of NO2 conversion to N2O5 in the polar night, and they suggest that downward transport in the mesosphere can be quite localized. The data represent the first experimental evidence that the thermosphere is an NOx source for the mesosphere and stratosphere.