A non-LTE radiative transfer model for the population of the CO(1) vibrational state in the middle atmosphere is presented. Radiative transfer is treated by using the Curtis matrix method. The transmittances needed for the Curtis matrix have been computed by using the GENLN2 line-by-line radiance/transmittance code. A study of the radiative and collisional processes and atmospheric parameters that affect the population of CO(1) populations has been performed. We find that the tropospheric kinetic temperature and CO abundance profiles play an important role through radiative coupling in the population of mesospheric CO(1) at night-time. During the day, the solar flux and the mesospheric CO abundance are the main factors determining the CO vibrational temperature in the mesosphere. Non-thermal collisions with the atmospheric N2(1) are also shown to be important for the daytime population of CO(1) in that region. The consequences of these results for the retrieval of CO abundances from the remote sensing of radiance at 4.7 μm are discussed. ¿ American Geophysical Union 1993