Phospholipase Dalpha1 (PLDalpha1) has been shown to mediate the abscisic acid regulation of stomatal movements. Arabidopsis plants deficient in PLDalpha1 increased, whereas PLDalpha1-overexpressing tobacco decreased, transpirational water loss. In the early stage of drought, the decrease in water loss was associated with a rapid stomatal closure caused by a high level of PLD in PLDalpha1-overexpressing plants. However, in the late stage of drought, the overexpressing plants displayed more susceptibility to drought than control plants. PLDalpha1 activity in the overexpressing plants was much higher than that of control plants in which drought also induced an increase in PLDalpha1 activity. The high level of PLDalpha1 activity was correlated to membrane degradation in late stages of drought, as demonstrated by ionic leakage and lipid peroxidation. These findings indicate that a high level of PLDalpha1 expression has different effects on plant response to water deficits. It promotes stomatal closure at earlier stages, but disrupts membranes in prolonged drought stress. These findings are discussed in relation to the understanding of PLD functions and potential applications.