Oxygen uptake in the dark was measured in slices cut from wheat flag leaves harvested at the end of the night or after a period of several hours in the light, in the presence and absence of KCN salicylhydroxamic acid (SHAM), sucrose or uncoupler. In end-of-night leaf slices, endogenous sugars and respiratory rates were relatively low and oxygen uptake occurred only via the cytochrome path. In slices from leaves in the light, endogenous sugar levels and respiratory rates were higher, and this increase was due in part to engagement of the alternative path. Carbonyl cyanide p-trifluoromethoxy phenyl hydrazone (FCCP) did not stimulate oxygen uptake rates per se in slices from leaves in the light, but stimulated flux through the cytochrome path at the expense of that through the alternative path. When both FCCP and sucrose were added together to slices from leaves in the light, oxygen uptake was stimulated, and this stimulation was due to engagement of the alternative path. We conclude from these results that wheat leaf respiration in the dark is regulated by both intracellular sugar levels and by adenylate control of the mitochondrial respiratory chain. When leaf sugar levels are substantial, the alternative path becomes engaged because the cytochrome chain is restricted. When leaf sugar levels are low, respiration is limited by substrate supply to the mitochondria, and the alternative path is not expressed.