Leaves of the reed grass (Arundo donax L.) were shown to emit the hydrocarbon isoprene (2-methyl-1,3-butadiene). To explore the role of photorespiration in isoprene biogenesis and emission simultaneous measurements were made of isoprene emission rate and leaf photosynthesis rate as photorespiration inhibitors were added through the transpiration stream. These measurements were also made when photorespiration was inhibited by decreasing the O₂ concentration in the leaf chamber. Inhibition of photorespiration under these conditions was verified separately. Fluoride, bisulfite, α-hydroxy-2-pyridinemethanesulfonic acid and isonicotinic acid hydrazide inhibited photorespiration 24-89% and isoprene emissions 26-96%. However, they also inhibited photosynthesis 26-78%, demonstrating that they are not specific photorespiration inhibitors. Photorespiration inhibition by more specific inhibitors, aminoacetonitrile (29% inhibition) or 2% O₂ concentration (94% inhibition), led to increases in isoprene emission rates. Varying the O₂ content in the gas stream demonstrated that isoprene emissions parallel increases or decreases in photosynthesis rates. These results demonstrate that isoprene biosynthesis and emission are independent of photorespiration.