The effect of O(2) on the CO(2) exchange of detached soybean leaves was measured with a Clark oxygen electrode and infrared carbon dioxide analysers in both open and closed systems.The rate of apparent photosynthesis was inhibited by O(2) while the steady rate of respiration after a few minutes in the dark was not affected. Part of the inhibition of apparent photosynthesis was shown to be a result of increased photorespiration. This stimulation of photorespiration by O(2) was manifested by an increase in the CO(2) compensation point.The differential effects of O(2) on dark respiration (no effect) and photorespiration (stimulation) indicated that these were 2 different processes.Moreover the extrapolation of the CO(2) compensation point to zero at zero O(2) indicated that dark respiration was suppressed in the light at least at zero O(2) concentration.