Concern is growing for the environmental water quality of the Great Lakes. Atmospheric deposition of toxic substances is recognized as a major pathway of contaminants to the water medium. To estimate the annual atmospheric loadings of five toxic metals--arsenic (As), cadmium (Cd), chromium (Cr), lead (Pb), and nickel (Ni)--to Lake Michigan, the Regional Lagrangian Model of Air Pollution (RELMAP) was applied using a preliminary 1985 airborne toxics emissions inventory developed by EPA for U.S. and Canadian anthropogenic sources. At 3-h intervals the model creates pollutant puffs containing particles with diameters of either 0.5 micrometers or 5.0 micrometers, transports them across the eastern North American domain, and calculates wet and dry deposition amounts for each unit-degree cell. Total direct deposition amounts to the lake are determined from these calculated amounts and a land-use inventory that defines the water surface portion of each cell.