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 extensive meteorological and oxidant database being generated concurrently by the Lake Michigan Ozone Study (LMOS). Integrated 12-hour atmospheric samples were collected daily from July 8 through August 9, 1991 at three ground sites (two collocated with LMOS stations). Over 1,200 samples were analyzed to determine atmospheric levels of PCBs, pesticides, PAHs, VOCs, particle mass, and trace elements (including mercury). In addition, a research vessel and a small aircraft were employed on selected days to measure micrometeorological parameters, pollutant concentrations and some fluxes at offshore locations near Chicago. The major goals of the 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.