The stable carbon isotope composition of isoprene emitted from leaves of red oak (Quercus rubra L.) was measured. Isoprene was depleted in (13)C relative to carbon recently fixed by photosynthesis. The difference in isotope composition between recently fixed carbon and emitted isoprene was independent of the isotopic composition of the source CO(2). beta-Carotene, an isoprenoid plant constituent, was depleted in (13)C relative to whole leaf carbon to the same degree as isoprene, but fatty acids were more depleted. Isoprene emitted from leaves fed abscisic acid was much less depleted in (13)C than was isoprene emitted from unstressed leaves. We conclude that isoprene is made from an isoprenoid precursor that is derived from acetyl-CoA made from recent photosynthate. The carbon isotope composition of isoprene in the atmosphere is likely to be slightly more negative (less (13)C) than C(3) plant material but when plants are stressed the isotopic composition could vary.