Several important biochemical functions in an epiphytic lichen, Evernia mesomorpha, in response to controlled SO2 exposures were studied. Appreciable changes in these functions were observed even at a very low SO2 concentration (0.1 ppm), suggesting that epiphytic lichens are probably one of the most sensitive species to SO2 exposures. Fumigation of these lichens at 0.34 ppm SO2 for 3 to 7 d caused irreversible injury to nearly all the biochemical functions studied. Controlled fumigation of vascular species with low levels of SO2 and NO2 caused appreciable changes in several biochemical and physiological functions; in many cases such exposures did not produce any visual symptoms of pollutant toxicity. Biochemical indicators such as peroxidase and glycol late oxidase exhibited a synergistic response and ribulose diphosphate carboxylase an additive response to an SO2 -NO2 mixture in alder. These enzymes, when examined from pine and birch, produced little or no additional response to the mixture compared to that produced by either pollutant individually. Metal pollutants such as vanadium and nickel were very toxic to various cellular processes. The maximum biochemical response was obtained by the metals alone followed by SO2. The pollutant mixture (metal + SO2 ) did not produce much more additional response than that caused by metals alone. Biochemical and physiological methods developed in the laboratory were utilized for detecting previsual air pollutant injury to jack pine and highly sensitive epiphytic lichens in the oil sands area. The biochemical functions examined showed no significant differences between the sites at different distances from the pollutant source. The lack of response is considered to be due to the ability of vegetation to recover its metabolic functions between the rare incidents of heavy fumigation. It is possible that the minor pollutant effects, if any, may be masked by the natural variability between sites.