We present a global chemical transport model called the Integrated Massively Parallel Atmospheric Chemical Transport (IMPACT) model. This model treats chemical and physical processes in the troposphere, the stratosphere, and the climatically critical tropopause region, allowing for physically based simulations of past, present, and future ozone and its precursors. The model is driven by meteorological fields from general circulation models (GCMs) or assimilated fields representing particular time periods. It includes anthropogenic and natural emissions, advective and convective transport, vertical diffusion, dry deposition, wet scavenging, and photochemistry. Simulations presented here use meteorological fields from the National Center for Atmospheric Research (NCAR) Middle Atmospheric Community Climate Model, Version 3 (MACCM3). IMPACT simulations of radon/lead are compared to observed vertical profiles and seasonal cycles. IMPACT results for a full chemistry simulation, with approximately 100 chemical species and 300 reactions representative of a mid-1990s atmosphere, are presented. The results are compared with surface, satellite, and ozonesonde observations. The model calculates a total annual flux from the stratosphere of 663 Tg O3/year, and a net in situ tropospheric photochemical source (that is, production minus loss) of 161 Tg O3/year, with 826 Tg O3/year dry deposited. NOx is overpredicted in the lower midlatitude stratosphere, perhaps because model aerosol surface densities are lower than actual values or the NOx to NOy conversion rate is underpredicted. Analysis of the free radical budget shows that ozone and NOy abundances are simulated satisfactorily, as are HOx catalytic cycles and total production and removal rates for ozone.