Within a conceptual framework of stratospheric injection, CO-CH4 background tropospheric chemistry, parameterized pollution production in the continental boundary layer and surface deposition, we use an 11 level GCTM to simulate global distributions of present and pre-industrial tropospheric O3. The chemistry is driven by previously simulated present and preindustrial NOx fields, while prescribed fields of CO, CH4 and H2O are held constant. An evaluation with measurements from 12 surface sites, 21 ozonesonde sites and 1 aircraft campaign finds agreement within ¿25% for 73% of the observations while identifying systematic errors in the wintertime high-latitude Northern Hemisphere (NH), the Southern Hemisphere (SH) tropics during biomass burning, and the remote SH. We predict that human activity has increased the annual integral of tropospheric ozone by 39% with 3/4's of that increase in the free troposphere, though the boundary layer [BL> annual integral has increased by 66%. The 2 largest components of the global O3 budget are stratospheric injection at 696 TgO3/yr, and loss through dry deposition, which increases from 459 TgO3/yr to a present level of 825 TgO3/yr. While tropospheric chemistry's net contribution is relatively small, changing from a preindustrial destruction of -236 TgO3/yr to a present production of +128 TgO3/yr, it is a balance between two much larger terms, -558 TgO3/yr of destruction in the background troposphere and +686 TgO3/yr of production in the polluted boundary layer. Human impact on O3 predominates in the summertime extratropical NH and in the tropics during their biomass burning seasons [increases of 50%--100% or more>. Conversely, there has been little increase in most of the upper troposphere [<20%>, where ozone's influence on tropospheric climate is strongest.¿ 1997 American Geophysical Union