Measurements of ozone and PAN (peroxyacetic nitric anhydride) were made at four sites in eastern North America; Bondville, Illinois, Egbert, Ontario, Scotia, Pennsylvania, and Whitetop Mountain, Virginia., in July and August of 1988 as part of a study of regional oxidant photochemistry. The concentrations of PAN ranged from <0.010 to 9.2 parts-per-billion by volume (ppbv) and those of O3 ranged from <2 to 139 ppbv. Diurnal concentration profiles showed PAN and O3 to be removed within nocturnal boundary layers, especially if nitric oxide was present, and that O3 was, for most sites, more rapidly removed than PAN. The only mountain top site at which PAN was measured showed distinctly different diurnal profiles in which O3 was actually higher at night, suggesting that convective flow driven by surface cooling served to transport air down from higher in the mixed layer at night. The afternoon production of PAN and O3 was also apparent and led to an overall linear correlation of O3 and PAN between the hours of 1300 and 1800. The comparison of this result with other measurements, estimates of PAN formation rates, and model calculations indicates that the production of PAN relative to ozone is far greater (¿3) than can be accounted for by acetaldehyde reactions with OH, there is a wide disparity in model descriptions of PAN production, and confirms the non-linear dependence of O3 production on NOX. ¿ American Geophysical Union 1995