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1. The observations of van der Veen (1949) that light curves for photosynthesis of leaves [P = f(I)] exhibit a sharp break (the Kok-phenomenon) at or close to the compensation point, have been checked. The method used was the same as that used by van der Veen. 2. Under normal conditions (20°C. concentrations of carbon dioxide of up lo 3.0 vol %) we failed lo demonstrate the presence of such breaks in all cases and in all species of plants tested. Throughout the range of light intensities studied by van der Veen our light curves were found to be absolutely linear indicating that within wide limits the quantum efficiency is independent of the light intensity. The conception of the existence of a Kok-phenomenon in photosynthesis of leaves (shift of quantum efficiency at the compensation point) cannot be supported. 3. In air with high concentrations of carbon dioxide (3.0 % 5.6 %) it was possible at 5° and at 10°C to observe light curves bending towards the axis of abscissas at low light intensities. In air with 9.9 % and 20 % CO2 the curves were observed to bend at 20°. Some of the bent curves show the presence of breaks, similar to those found in the curves of van der Veen. 4. The photosynthesis reducing effect of excess carbon dioxide thus demonstrated increases with increasing CO2-concentrations and decreases with increasing temperatures. It is fully reversible and is suspendet at higher temperatures or al lower concentrations of carbon dioxide. It may be compared to a narcotic poisoning. 5. The narcotic effect affects the dark reactions of photosynthesis exclusively; the mechanism of the narcotization is discussed. 6. The occurrence of breaks in the light curves of van der Veen may possibly be explained by the fact that a low temperature was used in the experiments in connection with a relatively high concentration of carbon dioxide (3.6 %). [ABSTRACT FROM AUTHOR]