A new three-dimensional chemical transport model of the troposphere is presented. This model, named intermediate model of global evolution of species, has been developed to study the global distributions, budgets, and trends of 41 chemical compounds, including the most important species that determine the oxidation capacity of the atmosphere. The chemical mechanism is made of approximately 125 chemical reactions and 26 photodissociations. The model accounts for surface emissions, chemical transformations, dry and wet deposition, and aerosol reactions of trace constituents. The model is formulated in &sgr; coordinates and includes 25 layers in the vertical. Its horizontal resolution is 5¿ in longitude and 5¿ in latitude. To keep the requirements in computer time limited, a simplified representation of the transport is adopted: the advection, solved by a semi-Lagrangian scheme, is driven by monthly mean climatological winds provided by an European Center for Medium-Range Weather Forecasts analysis. The effect of wind variability at timescales smaller than a month is taken into account by an eddy diffusion parameterization. Convection in cumulonimbus clouds is also represented. All input field, such as the distribution of winds, clouds, eddy diffusion coefficients, and the boundary conditions, are monthly means constrained by observational data. The modeled global distributions of species such as methane, carbon monoxide, nitrogen oxides, and ozone are generally in good agreement with observations. The lifetime of methane, which can be regarded as a measure of the oxidizing capacity of the atmosphere, is found to be equal to 11 years, in agreement with recent estimates. The model also shows that the deposition of ozone at the Earth's surface (1100 Tg/yr) balances the sum of the net photochemical production (550 Tg/yr) and the flux from the stratosphere (550 Tg/yr). In the case of carbon monoxide, surface emissions (1400 Tg/yr) are approximately 50% larger than in situ production by hydrocarbon oxidation (900 Tg/yr). ¿ American Geophysical Union 1995