Monoterpene levels in current year needles of Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings were measured at the end of 4 years of exposure to ambient or elevated CO2 (+179 µmol mol−1), and ambient or elevated temperature (+0.3.5^C). Eleven monoterpenes were identified and quantified using gas chromatography/flame ionization detector/mass spectroscopy, with eight of these compounds regularly occurring in all trees examined. Elevated CO2 exposure significantly reduced the levels for four of the eight main compounds in needles. Total monoterpene production was reduced by 52% (P < 0.05). Elevated temperature also reduced monoterpene levels (P < 0.07). The combination of elevated temperature and elevated CO2 resulted in a 64% reduction in total monoterpenes compared with needles on ambient temperature trees. Two-way anova showed no significant temperature-CO2 interaction. It is hypothesized that seasonal reductions in needle monoterpene pools under elevated CO2 and temperature conditions may be due to a combination of competing carbon sinks, including increased carbon flux through the roots.