Observations of carbon monoxide (CO) by the Measurement of Air Pollution from Space (MAPS) instrument onboard the space shuttle were assimilated into a global three-dimensional chemistry-transport model using the Physical-space Statistical Analysis System approach. The assimilation considerably improved the calculated distribution of CO in the troposphere. On the global scale, the adjustment of the CO field resulting from the assimilation procedure was large at the beginning of the assimilation, suggesting discrepancies in the initial conditions of the model. As the model integration/assimilation progressed with time, transport caused the model to drift. This drift limited to a few days the memory of CO from its adjustment toward the observations. The assimilation of CO significantly influenced the distribution of other chemical species, even over the limited time periods (~ 10 days) analyzed. ¿ 1999 American Geophysical Union