The partial pressure of intercellular CO2 derived from gas exchange data for water-stressed grapevine (Vitis vinijera L.), oleander (Nerium oleander L.) and red-flowering gum (Eucalyptus ficifolia F. Muell.) does not indicate the actual average intercellular CO2 of the leaf. This can be determined from chlorophyll fluorescence signals and modelled from conventional gas exchange information. Water stress increases endogenous levels of abscisic acid and the leaves respond in a similar manner to that observed when exogenous abscisic acid is applied. Autoradiograms show that water stress results in non-uniform gas exchange over small areas of the leaf. Non-uniform stomatal closure can account for the apparent non-stomatal inhibition of photosynthesis in plants experiencing water stress.