Observations at sites in eastern North America show a strong correlation between O3 and CO concentrations in summer, with a consistent slop ΔO3/ΔCO≈0.3. Observations in the aged Denver plume at Niwot Ridge, Colorado, also shows a strong correlation but with ΔO3/ΔCO=0.15. These data offer a sensitive test for evaluating the ability of photochemical models to simulate production of O3 over North America and its export to the global atmosphere. Application to the Harvard/Goddard Institute for Space Studies three-dimensional, continental-scale model shows that the model gives a good simulation of the observed O3-CO correlations and of the associated ΔO3/ΔCO. This successful simulation lends support to model estimates of 6 Gmol d-1 for the net O3 production in the U.S. boundary layer in summer (corresponding to a net O3 production efficiency of 5.5, which is the number of O3 molecules produced per molecule of NOx consumed) and 70% for the fraction of the net production that is exported to the global atmosphere. Export of U.S. pollution appears to make a significant contribution to total tropospheric O3 over the northern hemisphere in summer. Simple interpretation of observed ΔO3/ΔCO as an O3/CO anthropogenic enhancement ratio is shown to underestimate substantially anthropogenic O3 production, because O3 and CO concentrations are negatively correlated in the absence of photochemistry. It is also shown that concurrent observations of ΔO3/ΔCO and ΔO3/Δ(NOy-NOx) ratios can be used to impose lower and upper limits on the net O3 production efficiency. ¿ American Geophysical Union 1994