The effects of foliar applications of simulated acid rain (pH 5–6, 4.0, 3.0 and 2.0) were observed for seedlings of four deciduous tree species native to the eastern United States, Liriodendron tulipifera L., Platanus occidentalis L., Liquidambar styracifiua L., and Robinia pseudo-acacia L. Damage occurred only at pH 2.0. Liriodendron was least damaged, Platanus the most. For Liriodendron and Platanus, older leaves showed more damage than younger ones, whereas for Robinia the opposite was true. No age effect was noted for Liquidambar. Height growth was reduced at pH 20 in Platanus and Robinia, and less so in Liquidambar. Total biomass was significantly reduced at pH 2.0 for Platanus and Liquidambar, resulting mainly from reduction in stem and root biomass. Leaf biomass was not greatly affected in any of the species. Photosynthetic rates in Platanus were reduced after exposure to rain of pH 20, and were the result of changes in mesophyll conductance to CO2, as stomata! conductances were not affected. In Liquidambar, stomatal conductances to water vapour were lowered after exposure to rain of pH 2.0. Results show that under greenhouse conditions only rain of extremely low pH causes foliar damage and growth reductions. Lowered rates of photosynthesis in Platanus may have contributed to the observed reductions in growth.