AbstractThe effect of oxygen concentration on the rate of CO2‐uptake in continuous and intermittent light was studied as well as the CO2‐fixation during a short dark period after light was turned off. In addition the dark respiration and the CO2‐compensation point of attached and detached corn leaves were determined. Leaves of 4 to 22‐day old plants were used as experimental material. A closed circuit system of an infrared carbon dioxide analyzer was employed to measure the rate of CO2‐exchange. It was found that in an atmosphere consisting of 100 % oxygen, there was about 50 per cent inhibition of the rate of CO2‐uptake in continuous and intermittent light compared to that in an atmosphere consisting of 21% oxygen. The same was true of the rate of CO2‐fixation in darkness during a short period after the light was turned off. Since the response to oxygen concentration of the CO2‐uptake in light and of the CO2‐fixation in darkness after the light was turned off were similar, it is concluded that the fixation of CO2in the short dark period represents an over‐ shoot of photosynthesis. The rate of dark respiration was little affected by the oxygen concentration in the ranges used in the experiments. The carbon dioxide compensation point which has been observed in leaves of 4 to 14‐day old plants was not influenced by either oxygen concentration or light intensity. Since the changes in the rate of CO2‐uptake due to changes in the concentration of oxygen and light intensity had no effect on the CO2‐compensation point, it is concluded that a reabsorption of respiratory CO2by photosynthesis could not account for the low value of this point. These results are interpreted as a further corroboration of the statement that the leaves of corn lack the process of photorespiration and that dark respiration is inhibited in light.It was observed that the rate of the CO2‐uptake gradually increased in plants which were from 4 to 22‐days old. The inhibitory effect of high concentration of oxygen on the rate of CO2‐uptake was relatively higher i