The emission of toluene from different plants was observed in continuously stirred tank reactors and in field measurements. For plants growing without stress, emission rates were low and ranged from the detection limit up to 2⋅10-16 mol⋅cm-2⋅s-1. Under conditions of stress, the emission rates exceeded 10-14 mol⋅cm-2⋅s-1. Exposure of sunflower (Helianthus annuus L. cv. Gigantheus) to 13CO2 resulted in 13C-labeling of the emitted toluene on a time scale of hours. Although no biochemical pathway for the production of toluene is known, these results indicate that toluene is synthesized by the plants. The emission rates of toluene from sunflower are dependent on nutrient supply and wounding. Since &agr;-pinene emission rates are also influenced by these factors, toluene and &agr;-pinene emissions show a high correlation. During pathogen attack on Scots pines (Pinus sylvestris L.) significant toluene emissions were observed. In this case emissions of toluene and &agr;-pinene also show a good correlation. Toluene emissions were also found in field experiments with pines using branch enclosures. ¿ 1999 American Geophysical Union