In this first paper of a proposed series, it is shown that at a high light intensity all plants studied reached the same limiting CO2 concentration and maintained this level for many hours of continuous illumination. Ten species of widely different types of leafy plants have been used, and at low temperatures these behaved alike. It appears, therefore, that there is an extremely narrow range of CO2 concentration which so greatly limits the rate of photosynthesis that differences in rates of respiration and other internal factors have little effect on this point of balance between respired and assimilated CO2. Over a wide range this CO2 balance is independent of temperature. At very high temperatures (36° C), however, some species no longer maintain the usual CO2 balance, but reach a much higher value (figs. 6, 8). This is especially emphasized here because of its relation to the abnormal photosynthetic behavior of plants reported recently by Kostytschew and coworkers (8, 9).