The leaves of Primula palinuri and Zea mays are amphistomatic. The CO2-exchange of both leaf surfaces was measured separately. The distribution of stomata and the portion of CO2-uptake of both leaf surfaces correspond well. In Primula the greater part diffuses through the upper surface of the leaf, in Zea through the lower surface. Stripping of the Primula epidermis does not change the total CO2-uptake but the greater part now diffuses through the lower stripped surface of the leaf. The CO2-uptake of the upper surface alone, as well as of the lower surface alone and of both surfaces together was measured. The quantities of CO2 transported through the leaf surfaces are not independent of each other: if CO2-uptake is only possible through one surface of the leaf the CO2 flux on this side increases. The significance of this property of amphistomatic leaves in relation to transpiration and CO2 assimilation is discussed.