An airborne millimeter-wave sensor has been used to determine the verticle H2O profile over the altitude range of 20--70 km by measuring the thermal emission of the rotational transition at 183.31 GHz. Measurements were made between 45¿ N and 75¿ N during several flights in December 1986. At mid-latitudes they showed water vapor mixing ratios between 3.5 and 5 ppmv with a maximum at 50 km altitude. At high latitudes a decrease to values less than 2.5 ppmv in the lower mesosphere and a maximum at 30km was found. The characteristic decrease at high latitudes in the lower mesosphere in winter corresponds well with recent model calculations and is assumed to be caused by strong downward directed transport of mesospheric air. ¿ American Geophysical Union 1988