We report two single-day spectral observations of the terrestrial J=1 → 0 CO emission line (January 28, 1982, and April 18, 1984). We use these observations to deduce the mesospheric CO mixing ratio profile. The measurements were obtained by using the 7-m Cassegrain antenna located at the AT&T Bell Laboratories (millimeter-wave) Observatory in Holmdel, New Jersey (latitude, 40.4¿). The CO mixing ratios in the lower mesosphere inferred from the January 1982 observation are larger by a factor of 10-100 than those indicated by previous measurements, while the April 1984 observation is in good agreement with previous work. Thus our measurements indicate that the abundance of CO in the mesosphere can be extremely variable.