Using potted plants in a glasshouse, the relationship between ultrasound emission rate and water potential was investigated before, during and after a period of simulated drought. At the start of the experiment, just before budbreak, the plants emitted ultrasound pulses at rates of up to 25 c.p.m. During drought the rate increased to 60 c.p.m. Droughted plants displayed lower xylem density than the controls; and on this basis it was estimated that a third of the tracheids had emptied as a result of cavitation. On rewatering, the density of the xylem recovered and, after recovery, the plants that had been droughted failed to produce ultrasound emissions when water potential fell. By this time, the control plants, which had been well watered all the time, produced only a small number of ultrasound emissions when water potential fell.