The influence of oxygen concentration in the range 0-21 % on photosynthesis in intact leaves of a number of higher plants has been investigated. Photosynthetic Co2 fixation of higher plants is markedly inhibited by oxygen in concentrations down to less than 2%. The inhibition increases with oxygen concentration and is about 30% in an atmosphere of 21% O2 and 0.03 % CO2. Undoubtedly, therefore, oxygen in normal air exerts a strong inhibitory effect on photosynthetic CO2 fixation of land plants under natural conditions. The inhibitory effect of oxygen is rapidly produced and fully reversible. The degree of inhibition is independent of light intensity. The quantum yield for CO2 fixation, i.e. the slope of the linear part of the curve for CO2 uptake versus absorbed quanta, is inhibited to the same degree us the light saturated rate at all oxygen concentrations studied. Diverse species of higher plants, varying greatly in photosynthetic response to light intensity and CO2, concentration, and with light saturated rates of CO2 fixation differing by a factor of more than 10 times, show a remarkable similarity in their response to oxygen concentration. By contrast, when studied under the same conditions as the higher plants, the green algae Chlorella and Ulva did not show any measurable inhibition of photosynthetic CO2 fixation. Similarily, the increase in fluorescence intensity with increasing oxygen concentrations found in higher plants also was not seen in Chlorella. The Present results, together with previous data on the photosynthetic response of algae to oxygen concentration, indicate that the photosynthetic apparatus of higher plants differs considerable from that of algae in its sensitivity to oxygen. The inhibitory effect of oxygen on photosynthetic CO2 fixation in higher plants is somewhat higher at wavelengths which excite preferentially photosystem I. Also, the Emerson enhancement of CO2 fixation measured when a far red beam of low, intensity is imposed on a background of red light is greater under low oxygen concentration that under air. Measurements of reversible light-induced absorbance changes reveal that the change at 591 nm, probably caused by plastocyanin, is affected by oxygen concentration only if photosystem II is excited. The reducing effect on plastocynin, caused by excitation of this system, decreases with increasing oxygen concentration. From these results it is suggested that a possible site of the inhibition by oxygen is in the electron carrier chain between the two photosystems. Oxygen might act as an electron acceptor at this site, causing reducing power to react back with molecular oxygen. However, this hypothesis does not account for equal inhibitions of the quantum yield and light saturated rate of photosynthetic CO2 uptake. [ABSTRACT FROM AUTHOR]