Branches on mature red spruce trees (PicearubensSarg.) were fumigated with SO2in late summer of 1990 and 1991. There were four SO2treatment levels (0, 0.1, 0.2, and 0.4 ppm) and 12 fumigation chambers. Each treatment was replicated on three neighboring trees. Net photosynthesis and stomatal conductance of current-year foliage were determined with a Li-Cor photosynthesis system. Rates of foliar SO2absorption were estimated using regression models, with leaf temperature, relative humidity, atmospheric CO2concentrations, and photosynthetically active radiation as predictors. Fumigation effects became visible in terms of needle yellowing and browning after a few weeks of treatment. Effects on net photosynthesis and stomatal conductance were generally negative and in direct proportion with cumulative foliar SO2absorption. It was found that, in shade-adapted foliage, visible symptoms were delayed, and stomatal conductance was slightly stimulated at low SO2levels. In other foliage, leaf yellowing and browning occurred earlier, and stomatal conductance was depressed. Net photosynthesis was depressed at about the same rate regardless of branch position. Foliage subjected to high SO2concentration treatments did not recover from SO2damage 1 year after treatment: needles had fallen off the twigs, and needle and twig length of new foliage was reduced.