When detached maize leaves were fed with an ABA solution via the xylem, the relationship between the relative stomatal inhibition and ABA concentrations was similar under different humidity conditions, but the relationship between such inhibition and ABA flux was different according to changes of humidity. To understand whether such stomatal behaviour was related to the way through which xylem-delivered ABA was metabolized, detached leaves of maize and Commelina were fed with tritium-labelled (3H)-ABA at concentrations similar to that found in xylem of droughted plants and it was found that xylem-delivered ABA was metabolized rapidly in both species. The half-life of ABA metabolism, calculated from the time-related ABA disappearance curve, was 42 and 64 min for maize and Commelina, respectively. The very short half-life suggests that there is a large capacity in leaves to metabolize xylem-delivered ABA and that metabolism is a major factor in the control of ABA accumulation in leaves. When ABA was fed at different fluxes, either through changing the feeding concentrations or through manipulating the rates of leaf transpiration (i.e. the volume flux), ABA was metabolized at rates that were proportional to the amount that was delivered. The absolute rate of ABA metabolism was, therefore, linearly related to the amount of ABA that had arrived. It was found that xylem-delivered ABA reached the epidermis of Commelina, and was metabolized at the same pattern as that in mesophyll tissues, i.e. at a similar half-life and at rates constantly related to the amount that was delivered. The role of the rapid ABA metabolism was discussed in the context of stomatal control by either concentration or flux of xylem-carried ABA.