When roots of young maize seedlings are forced to penetrate soil of high mechanical impedance, abscisic acid (ABA) concentration in the xylem sap can be increased by up to 10-fold. After a further 8-10 d, however, this increase disappears nearly completely. Leaf growth rate was inhibited in plants in compacted soil and showed a good relationship with ABA concentration in the xylem. The early large increase of ABA in xylem coincided with the reduction of leaf water potential and turgor which also gradually disappeared when plants became established. With only one layer of compacted soil in the middle of the soil profile, a similar situation to that in compacted field plots, plant leaf growth was not affected as long as the soil was well watered.