During the last 20--30 years, noticeable changes have occurred in emissions related to anthropogenic activity over the globe. In this study a three-dimensional chemical transport model for the troposphere (the 3-D Oslo CTM1 model) is used to investigate how changes in the concentration of carbon monoxide (CO) and ozone (O3) in the period 1980--1996 can be related to changes in emission rates and patterns. The results indicate that changes in both species reflect the emission changes well, especially CO because it is to a large degree driven by CO emissions. The most pronounced effect of the changes in emissions for CO and O3 is seen in eastern Europe and Southeast Asia. The annual trend in CO and O3 concentrations in the planetary boundary layer during the period 1980--1996 in eastern Europe is -1.94¿0.42 ppbv/yr and -0.01¿0.02 ppbv/yr, respectively. In Southeast Asia a positive trend is seen, with 2.29¿0.13 ppbv/yr in the CO concentration and 0.68¿0.03 ppbv/yr in the O3 concentration. The agreement between the modeled CO and O3 concentrations and observations is good. For O3 an improvement is gained in the modeled results compared with observations, by including changes in emissions from aircraft, and two effects of reduced stratospheric O3 level (altered photolysis rates and stratospheric-tropospheric exchange) during the period of study. ¿ 2000 American Geophysical Union