The effects of exogenous abscissic acid (ABA) application on the antioxidant defenses were investigated in grass pea (Lathyrus sativus L.) seedlings. Four treatment combinations of ABA (1×10-3 nM ABA) and PEG (10% polyethylene glycol, PEG 6000) were designed to evaluate their short-term (48h) effect: (1) Well-watered group (Control group1), (2) PEG treatment, (3) ABA treatment, and (4) PEG+ABA treatment. In addition, 2 other treatments were used to evaluate the long-term (15d) effect of ABA: well-watered group (Control group2) and ABA treatment. Time-course analyses of ABA content, the production of malondialdehyde (MDA) and H2O2, and the activities of superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR) and peroxidase (POD) in water-stressed leaves showed that a significant increase in ABA content preceded that of MDA and H2O2 in long term experiment, which was followed by a substantial increase in the activities of four antioxidant enzymes. Under the short-term drought stress, ABA application promoted the activities of antioxidant enzymes, and reduced the accumulation of MDA and H2O2 significantly. On the contrary, long term application of exogenous ABA increase the generation of MDA and H2O2 significantly. It could be argued that during a successive application of exogenous ABA, ABA played a dual role by which the beneficial role in the initial stage shifted to a detrimental one under prolonged treatment in up-regulating protective defense strategies in plants.