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Drought is a major environmental stress that limits the growth and productivity of fruit trees in semiarid and arid regions. We evaluated the potential of exogenous nitric oxide (NO) to improve the drought tolerance of apple rootstocks (Malus spp.). Leaves of 2-year-old seedlings of drought-sensitive Malus hupehensis (Pamp.) Rehd. and drought-tolerant Malus sieversii (Ledeb.) M. Roem. rootstocks were sprayed with NO donor sodium nitroprusside (SNP) at 0-400 mu mol L-1, and then the plants were subjected to drought stress. Among all SNP treatments, the 300 mu mol L-1 SNP treatment mostly alleviated drought-induced ion leakage and the accumulation of malondialdehyde and soluble proteins in M. sieversii and M. hupehensis leaves. These changes helped to maintain leaf water potential and relative water content of the apple rootstocks under drought stress. The activities of several antioxidant enzymes in leaves increased under drought stress, whereas photochemical efficiency decreased. The adverse effects of drought were exacerbated by treatment with the NO scavenger cPTIO (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt; 400 mu mol L-1); however, this effect was offset by NO application. These results suggested that the NO donor SNP effectively protected Malus seedlings from drought-induced oxidative damage by enhancing antioxidant enzyme activities and photosynthetic performance.