The 1990 Clean Air Act Amendments include specific provisions to study significant sources of atmospheric deposition of toxic substances and their impacts on the health and welfare of the Great Lakes and other major water bodies. The act also requires an assessment of the atmospheric loadings to the Great Lakes and other major water bodies. However, because of cost and technological limitations, spatially-integrated atmospheric deposition to a body of water can not be directly determined. Until recently, the total annual atmospheric deposition was estimated from the product of (1) spatially-limited, land-based, rural air concentrations, (2) constant theoretical or empirical atmospheric removal rates, and (3) the surface area of the water body. Although it is expedient, this approach, with its inherent assumptions, has several serious drawbacks that cast doubts on the spatially-integrated atmospheric loadings. Comprehensive atmospheric deposition models based on future enhanced versions of currently-available regional air pollution models offer a more accurate alternative to this traditional approach. However, since a better understanding of the emission rates and atmospheric processes governing airborne toxic deposition is a prerequisite for the enhanced model versions, these comprehensive models will not be operational within the next several years. In the interim, the Regional Lagrangian Model of Air Pollution (RELMAP), a simple atmospheric deposition model, was applied to quantify the expected range of annual deposition amounts of toxic trace metals to Lake Michigan. These metals are arsenic (As), cadmium (Cd), chromium (Cr), lead (Pb), and nickel (Ni).