Two woody plants,Platycladus orientalise (tolerant to drought) andAcacia auriculiformis (sensitive to drought), have been subjected to rapid and slow soil drying. ABA levels in their roots and xylem sap have been determined using radioimmunoassay (RIA, sensitivity is 0.4 pmol per assay vial) with a monoclonal antibody against ( + )-ABA. ABA contents ofP. orientalise andA. auriculiformis growing in well watered soil are 0.3 and 2.5 nmol · g ow −1 in roots and 1.6 and 0.4 μmol in xylem saps, respectively. A rapid soil drying has been applied to these two plants with soil water content (SWC) being reduced to 0.02 and 0.06 g · g ow −1 respectively. Under such treatment, ABA was increased by 22 times and 2 times in roots and by 7 times and 34 times in xylem saps respectively forP. orientalise andA. auriculiformis. After rewatering for 6 d, ABA in roots and xylem sap of both species returned to control levels. When a slow soil drying was applied, SWC was reduced to 0.1 and 0.13 g · g OW −1 respectively forP. orientalise andA. auriculiformis. ABA was increased by 5 times and 1.6 times in roots and by 6 times and 19 times in xylem saps respectively for these two plants. ABA in roots and xylem saps decreased to near control levels 8 d after watering. Plant leaf water potentials of both plants hardly changed at times when root and xylem ABA showed substantial increase in response to soil drying. It is concluded that ABA levels in the roots and xylem saps ofP. orientalise andA. auriculiformis are more sensitively regulated than leaf water potential in response to soil drying and can act as a chemical signal in root-shoot communications of the drought stress.