This study was carried out in a greenhouse to investigate the effect of drought stress on changes of abscisic acid, water potential and relative water content of leaves in four pistachio seedling rootstocks in Tarbiat Modares University, Tehran, Iran during 2011-2012. Irrigation treatments: 100% (without stress), 65% (moderate stress) and 30% (severe stress) evapotranspiration requirements (ET_c), were applied for 4 months old pistachio seedlings for 75 days. Rootstocks were Pistacia vera ‘Badami-e-Zarand’, ‘Ghazvini, ‘Sarakhs’ and P. mutica. The parameters of leaf abscisic acid concentration, leaf water potential and relative water content (RWC) of leaves were evaluated. The results showed that water stress decreased the water potential and relative water content of the leaves while the abscisic acid concentration of the leaves increased. The concentration of abscisic acid in the Sarakhs increased significantly under drought stress conditions compared to other rootstocks, which can be said to be more sensitive under drought stress. P. mutica at all irrigation levels showed the lowest leaf water potential among the rootstocks. Leaf water potential of P. mutica at moderate and severe stress levels were -2.09 and -2.48 Mpa, respectively, while the difference among the other rootstocks in the case of leaf water potential was not significant. The P. mutica and Badami rootstocks had higher relative water content under stress conditions. Higher relative water content of P. mutica rootstock, in spite of lower water potential, can be attributed to better osmotic adjustment of this rootstock. Based on the reasons stated, it can be concluded that P. mutica and Pistacia vera ‘Badami-e-Zarand’rootstocks were more tolerant to drought stress than the other rootstocks.