In cotton the rate of CO2 assimilation, at O2 partial pressures up to 200 mbar, increased to a maximum and then declined as the intercellular partial pressure of CO2 was increased. The specific intercellular partial pressure of CO2 at which rate of assimilation began to decline depended on the environmental conditions. At 19 mbar partial pressure of O2 the decline occurred at CO2 partial pressure >390 µbar. At 200 mbar partial pressure of O2 it occurred at CO2 partial pressure > 534 µbar. O2 increased the CO2 partial pressure required for inhibition but it did not appear to affect the steepness of the decline of rate of assimilation with further increase in partial pressure of CO2 once the decline became apparent. The decline was more readily observed at low temperature and low O2 partial pressure, and in plants grown at low light and NO3- levels. It was also observed in cowpea and sunflower. Changes in quantum efficiency in cotton at high and low CO2 concentrations were observed. At ambient CO2 concentration (300 µbar), the quantum yields measured at 19 and 200 mbar partial pressure of O2 were 0.072 ± 0.0003 and 0.053 ± 0.0060 mol CO2 per mol absorbed quanta, respectively. In contrast, at 900 µbar CO2 partial pressure the respective values were 0.050 ± 0.0023 and 0.070 ± 0.0006 mol CO2 per mol absorbed quanta. The nature of the inhibition of CO2 assimilation by high partial pressure of CO2 is discussed.