The present experiment was designed to test the hypothesis that ABA is produced by the roots when they encounter drying soil and is transported to the shoot through the xylem sap to inhibit stomatal conductance quite independently of any change in the leaf water status. Sycamore seedlings were grown with their root systems divided equally between two containers. Water was withheld from one container while the other container was kept well-watered. Abscisic acid concentration in roots, xylem sap and leaves were evaluated. The role of xylem sap ABA as a sensitive indicator of the effect of soil drying is discussed