Isoprene is the most important nonmethane hydrocarbon emitted by plants. The role of isoprene in the plant is not entirely understood but there is evidence that it might have a protective role against different oxidative stresses originating from heat shock and/or exposure to ozone (O(3)). Thus, plants under stress conditions might benefit by constitutively high or by higher stress-induced isoprene emission rates. In this study, measurements are presented of isoprene emission from aspen (Populus tremuloides) trees grown in the field for several years under elevated CO(2) and O(3). Two aspen clones were investigated: the O(3)-tolerant 271 and the O(3)-sensitive 42E. Isoprene emission decreased significantly both under elevated CO(2) and under elevated O(3) in the O(3)-sensitive clone, but only slightly in the O(3)-tolerant clone. This study demonstrates that long-term-adapted plants are not able to respond to O(3) stress by increasing their isoprene emission rates. However, O(3)-tolerant clones have the capacity to maintain higher amounts of isoprene emission. It is suggested that tolerance to O(3) is explained by a combination of different factors; while the reduction of O(3) uptake is likely to be the most important, the capacity to maintain higher amounts of isoprene is an important factor in strengthening this character.