Spruce (Picea Abies L.) trees from the same clone were supplied with different, but low, amounts of plant available sulfate in the soil (9.7-18.1 milligrams per 100 grams of soil). Branches attached to the trees were enclosed in a dynamic gas exchange cuvette and analyzed for the emission of volatile sulfur compounds. Independent of the sulfate supply in the soil, H(2)S was the predominant reduced sulfur compound continuously emitted from the branches with high rates during the day and low rates in the night. In the light, as well as in the dark, the rates of H(2)S emission increased exponentially with increasing water vapor flux from the needles. Approximately 1 nanomole of H(2)S was found to be emitted per mole of water. When stomata were closed completely, only minute emission of H(2)S was observed. Apparently, H(2)S emission from the needles is highly dependent on stromatal aperture, and permeation through the cuticle is negligible. In several experiments, small amounts of dimethylsulfide and carbonylsulfide were also detected in a portion of the samples. However, SO(2) was the only sulfur compound consistently emitted from branches of spruce trees in addition to H(2)S. Emission of SO(2) mainly proceeded via an outburst starting before the beginning of the light period. The total amount of SO(2) emitted from the needles during this outburst was correlated with the plant available sulfate in the soil. The diurnal changes in sulfur metabolism that may result in an outburst of SO(2) are discussed.