During the summer of 1991, an air toxics monitoring program was conducted in the lower Lake Michigan area. The study was designed to take advantage of the intensive meteorological and oxidant data base being generated concurrently by the Lake Michigan Ozone Study (LMOS stations). Over 1,200 samples were collected and analyzed to determine atmospheric levels of PCBs, pesticides, PAHs, VOCs, particle mass, and trace elements. In addition, a research vessel and a small aircraft were employed on selected days to measure micro-meteorological parameters and pollutant concentrations at offshore locations near Chicago. The goals of this Great Waters pilot study were to evaluate methods of sample collection and analysis, quantify the atmospheric concentrations of toxic substances in the lower Lake Michigan area, compare measurements made over land and over water, attempt to differentiate the Chicago urban plume from regional background, identify categories of sources for the target pollutants, and estimate deposition to the lake.