We report on observations of the J=1→0 emission line of terrestrial mesospheric CO at 115 GHz. The measurements were made in the fall of 1993 and in February 1994 from Bern (47¿N, 7.5¿E). Column densities above 65 km as well as mesospheric vertical distributions were derived from the measured spectra. Between the end of September and middle November an increase by a factor of 2 of the column density was detected, in accordance with theory and previous observations. The February measurements exhibited a short-term variation on a timescale of a few days which may be explained by a combination of horizontal advection of air and gravity wave activity. ¿ American Geophysical Union 1995