Data on pH in free space, cytoplasm, chloroplasts and vacuoles in leaf tissue are used to calculate the distribution of abscisic acid (ABA) amongst these compartments, assuming that the intervening membranes are permeable to undissociated ABA only. Data on the permeability of membranes to ABA are used to calculate the time constant for equilibration between the free space and the other components. It is concluded that changes in pH in the chloroplast stroma due to irradiance, or other factors, will change the amount of ABA available to the guard cells via the free space, and that the time constant is similar to that for light-induced stomatal movement. The possibility that such changes play a role in modulating stomatal aperture is discussed.