The 3-week exposure of a branch of a forest-grown red spruce (Picea rubens) sapling to the combination of gaseous hydrogen peroxide, ozone, and sulfur dioxide is reported on. The exposure was conducted continuously using concentrations of H2O2, O3, and SO2 that have been observed during the summertime on the summit of Whiteface Mountain, New York. Fluxes of H2O, CO2, and the three pollutants were determined throughout the exposure. At weekly intervals, measurements of chlorophyll fluorescence, stomatal conductance, and hydrocarbon emissions were made. The response of the branch was compared to an equivalent branch of the same tree which received no pollutants but was otherwise treated identically. The exposure produced no visible injury symptoms but did produce an increase in dark respiration; the respiration rate more than doubled during the 21-day exposure period. Net photosynthesis was unaffected for both the experimental and the control branches. Nighttime fluxes of SO2 and H2O2 to external plant surfaces were significant. The stomatal component of O3 uptake by the branch displayed a linear increase during the experiment, and showed no evidence of saturating. Daytime and nighttime fluxes of H2O2 were increasing at the end of the experiment. Itwas observed that isoprene is emitted from red spruce, but saw no clear-cut change in emission rate in response to the exposure experiment.