An attempt was made to determine the relative importance of the stomata in restricting CO2 uptake under conditions of water stress. The air-phase and liquid-phase resistances to uptake of CO2 were determined from measurements of the rates of assimilation and transpiration and the corresponding concentration gradients of CO2 and H2O vapour. The results showed that the air-phase resistances (stomata) could account for only half the reduction in the rate of photosynthesis accompanying water stress. Experiments in which air was passed through the leaf confirmed that water stress restricted CO2 fixation within the leaf itself. The results and their possible explanation are discussed in relation to other work.