The influence of light or darkness on stomatal opening in epidermal strips of Commelina benghalensis was evaluated in the presence or absence of O(2) and/or metabolic inhibitors. Opening was restricted in nitrogen and was promoted by NADH and acids of the tricarboxylic acid cycle (succinate and alpha-ketoglutarate) in CO(2)-free air in light as well as in darkness. The enhancement by light of stomatal opening was prevalent under nitrogen or in the presence of the respiratory inhibitors (sodium azide and oligomycin). Respiratory inhibitors decreased the opening in light or darkness under CO(2)-free air but exhibited no effect under nitrogen, whereas phosphorylation uncouplers were inhibitory in light or darkness under both CO(2)-free air and nitrogen. The results suggest that oxidative phosphorylation is a basic source of energy for stomatal opening, although photophosphorylation could be an energy source.