Ogawa passive O 3 samplers were used in a 13-week study (June 1-September 1, 1999) involving 11 forested and mountaintop sites in north-central Pennsylvania. Four of the sites were collocated with TECO model 49 O 3 analyzers. A significant correlation (p < 0.0001) was found for 24-hr average weekly O 3 concentrations between the two methodologies at the four sites with collocated monitors. As expected, there were positive relationships between increasing elevation of the sites and increasing O 3 concentrations. No O 3 exposure patterns were found on a west-to-east or south-to-north basis; however, the area known for lower O 3 exposures within a smaller subsection of the study area showed consistently lower O 3 exposures. Preliminary results regarding relationships of symptom responses within O 3 -sensitive bioindicators are also presented with black cherry (Prunus serotina, Ehrh.) and common milkweed (Asclepias syriaca, L.) showing clear evidence of increasing injury with increasing O 3 exposures. Due to the extremely dry conditions encountered in north-central Pennsylvania during the 1999 growing season, O 3 -induced symptoms were sporadic and quite delayed until late-season rains during the latter portion of the observation period.