Continuous measurements of peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), and ozone (O3) were made from June 1997 through August 1999 at a ground site in the upper Midwest as part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET). Relatively clean air originating in Canada is transported to the site from the northwest, while more polluted air originating in the metropolitan regions of the Midwest is transported mainly from the southwest and the southeast. Monthly averages of PAN are a minimum in summer/early winter (July--January) with mixing ratios ranging from 125 to 259 parts per trillion by volume (pptv), while maximum monthly average PAN levels are observed in February through June with mixing ratios of 325--390 pptv. Monthly average ozone mixing ratios ranged from 42 to 46 ppbv in the spring/summer months and 23--32 ppbv in the autumn/early winter months. Correlation among species were examined on a monthly basis. There is a dramatic variation in the PPN/PAN slope from summer to winter with the general trend a maximum slope in the winter ranging from 0.20 to 0.23 and a minimum slope during the summer months ranging from 0.09 to 0.13. The degree of association, r2, is strong in all months, ranging from 0.76 to 0.96. A photochemical box model was used to determine if the seasonal variation could be attributed solely to the summertime production of PAN from isoprene. Box model results indicate that the seasonal variation in the PPN/PAN slope when produced from anthropogenic precursors can be explained by seasonal changes in temperature and photolysis rates, and the production of PAN from isoprene in the summer only serves to increase the seasonal difference in slope. ¿ 2001 American Geophysical Union