It was found for two species that net carbon dioxide uptake rates were reduced at constant intercellular carbon dioxide partial pressure when single attached leaves were exposed to large leaf to air water vapour pressure differences. Leaf temperature, irradiance, and ambient carbon dioxide and oxygen partial pressures were kept constant. Net carbon dioxide uptake rates decreased linearly with increasing vapour pressure difference, even in cases where transpiration rates were highest at intermediate values of vapour pressure difference. Decreases in net carbon dioxide uptake rates were quickly reversible. Different wind speeds across the measured leaf, different vapour pressure deficits around the rest of the shoot, and transient responses of net carbon dioxide uptake rate to abrupt changes in vapour pressure difference all gave the same response of net carbon dioxide uptake rate to vapour pressure difference. The data show that the inhibition of net carbon dioxide uptake rate at a given vapour pressure difference was not simply related to whole leaf transpiration rate or stomatal conductance.