Xylem sap from unwatered maize plants was collected and tested for antitranspirant activity. Two assays were used. These were a transpiration assay with detached wheat leaves and a stomatal bio-assay involving the direct microscopic observation of epidermis of Commelina communis. The reduction in transpiration of detached wheat leaves promoted by xylem sap could be duplicated almost exactly by the application of solutions of ABA of equivalent concentration to that found in the xylem sap. Removal of virtually all the ABA from the xylem sap, using an immunoaffinity column, removed virtually all the antitranspirant activity in both assays. These results are discussed in the context of other results which suggest the presence of as-yet unidentified inhibitors in the xylem sap of unwatered plants. We suggest that with maize plants at least, stomatal responses to soil drying can be entirely explained by enhanced concentration of ABA in the xylem stream.