The objective of the Tropospheric Ozone (O3) Global Model Intercomparison Exercise performed in 1997 was to systematically evaluate the capabilities of the current generation of 3-dimensional global models to simulate tropospheric ozone and their precursor gases, and to identify key areas of uncertainty in our understanding of the tropospheric O3 budget. This exercise has been organised by Global Integration Modelling (GIM) Activity of the International Global Atmospheric Chemistry (IGAC) Project. The present paper focuses on the capability of the models to simulate carbon monoxide, which is an important pollutant in the troposphere. The intercomparison of twelve 3-dimensional global chemistry/transport models shows significant differences between the models although all of them capture the general patterns in the global distribution of CO.