Carbon dioxide concentrations due to fossil fuel burning and CO2 exchange with the terrestrial biosphere have been modeled with 12 different three-dimensional atmospheric transport models. The models include both on-line and off-line types and use a variety of advection algorithms and subgrid scale parameterizations. A range of model resolutions is also represented. The modeled distributions show a large range of responses. For the experiment using the fossil fuel source, the annual mean meridional gradient at the surface varies by a factor of 2. This suggests a factor of 2 variation in the efficiency of surface interhemispheric exchange as much due to differences in model vertical transport as to horizontal differences. In the upper troposphere, zonal mean gradients within the northern hemisphere vary in sign. In the terrestrial biotic source experiment, the spatial distribution of the amplitude and the phase of the seasonal cycle of surface CO2 concentration vary little between models. However, the magnitude of the amplitudes varies similarly to the fossil case. Differences between modeled and observed seasonal cycles in the northern extratropics suggest that the terrestrial biotic source is overestimated in late spring and underestimated in winter. The annual mean response to the seasonal source also shows large differences in magnitude. The uncertainty in hemispheric carbon budgets implied by the differences in interhemispheric exchange times is comparable to those quoted by the Intergovernmental Panel on Climate Change for fossil fuel and ocean uptake and smaller than those for terrestrial fluxes. We outline approaches which may reduce this component in CO2 budget uncertainties. ¿ American Geophysical Union 1996