On average, a doubling in current atmospheric [CO(2)] results in a 15 to 20% direct inhibition of respiration, although the variability associated with this value is large within and among species. Direct effects of CO(2) on respiration may also be relevant to tree canopies because of dynamic fluctuations between nighttime and daytime [CO(2)] throughout the growing season. The mechanism by which CO(2) inhibits respiration is not known. A doubling of ambient [CO(2)] inhibits the activity of the mitochondrial enzymes, cytochrome c oxidase and succinate dehydrogenase. If inhibition of these enzymes is the only factor involved in the direct inhibition of respiration, the overall inhibition of specific respiration will be proportional to the control that such enzymes exert on the overall respiratory rate. We analyzed the effects of [CO(2)] on respiration in an attempt to scale the direct effects of CO(2) on respiratory enzymes to the whole-plant level. Sensitivity analysis showed that inhibition of mitochondrial enzymes by doubling current atmospheric [CO(2)] does not explain entirely the CO(2) inhibition of tissue or whole-plant respiration. We conclude that CO(2)-dependent suppression of respiratory enzymatic activity will be minimal when cytochrome c oxidase inhibition is scaled up from the mitochondria to the whole tree and that the primary mechanism for the direct inhibitory effect remains to be identified.