ABA exogenously applied to the leaves of the whole plants of pear (Pyrus bretschneideri Redh. cv. Suly grafted on Pyrus betulaefolia Rehd.) significantly increased the betaine concentrations in the leaves when the plants were well watered. The plants subjected to 'drought plus ABA' treatment had significantly higher betaine concentrations in their leaves than those given drought treatment alone. The 'drought plus ABA' treatment increased the amount of betaine aldehyde dehydrogenase (BADH, EC 1.2.1.8) and its activity in the leaves more than did the drought treatment alone. The experiments with detached leaves showed that ABA treatment significantly increased the concentration of betaine, activity of BADH and apparent amount of BADH in non-dehydrated leaves, and enhanced the accumulation of betaine, activity of BADH and apparent amount of BADH in dehydrated leaves. These effects of ABA were both time- and dose-dependent. Two ABA isomers, (-)-cis, trans-ABA and 2-trans, 4-trans-ABA, had no effect on the betaine accumulation in the leaves, showing that the ABA-induced effects are specific. These data demonstrate that ABA is involved in the drought-induced betaine accumulation in the pear leaves.