Seedlings of red spruce (PicearubensSarg.) were exposed repeatedly, either in the field or in the greenhouse, to sulfuric acid mist, nitric acid mist (with or without ammonium ions), or a combination of sulfuric and nitric acid mist (with or without ammonium ions) for 10 to 39% of the time over periods of 6 to 19 weeks. Sulfuric acid mist produced visible foliar injury after repeated overnight exposures to pH 3 or less. Foliar symptoms were significantly less or absent after exposures to nitric acid or combined sulphuric–nitric acid mist at the same pH values. Abscission of needles during the subsequent winter also was increased by sulfuric acid mist. Intermittent exposures that contained fewer hours of treatment with acidic mist and longer drying periods than continuous exposures caused greater amounts of foliar injury. Wind during the drying periods enhanced foliar injury, especially with sulfuric acid mist. Significant reductions in shoot elongation occurred with sulfuric acid mist in those experiments where seedlings continued to grow throughout the treatment period. Different provenances responded similarly to acidic mist. The results indicate that red spruce is particularly vulnerable to injury from acidic mist when sulfate concentrations are high and when mist exposures are intermittent with repeated periods of drying of liquid on foliage.