Objective To investigate the effects of nitric oxide(NO) and its donor(L-arginine) on focal cerebral ischemic injury in rats. Methods Forty-two healthy male SD rats weighing 250-300g were used. The animals were anesthetized with 20% urethane 1g· kg-1. Common carotid artery (CAA), external carotid artery(ECA) and internal carotid artery were(ICA) exposed through a median incision in the neck. Middle cerebral artery occlusion (MCAO) was produced by insertion of a 40 mm long nylon thread (0.3 mm in diameter) into ICA through ECA. The tip of the nylon thread was made into a ball of 0.5 mm in diameter with heat and the length of insertion was (18.5±0.5)mm on average. The animals were randomly divided into four groups: group 1: sham operation; group 2: ischemia group; group 3: low dose L-arginine(300mg·kg-1 intraperitoneal injection) and group 4: high dose L-arginine(500mg·kg-1 IP). Group 3 and 4 were further divided into 3 subgroups: subgroup A: L-arginine was given 1h after MCAO and the animals were killed 2h after medication (1h + 2h); subgroup B: L-arginine was given 3h after MCAO and the animals were killed 3h after medication(3h + 3h) and subgroup C: L-arginine was given 6h after MCAO and the animals were killed 3h after medication(6h + 3h) . The brain was removed immediately. The volume of cerebral infarct/volume of whole brain was calculated(%) . The NO, MDA content and NOS, SOD activity in the brain tissue of ischemic hemisphere were measured. Results L-arginine significantly decreased cerebral infarct volume and ameliorated focal cerebral ischemia. The effects in high dose group were better than in low dose group. L-arginine significantly increased NO content, decreased MDA content and enhanced the SOD activity in the focal ischemic cerebral tissue. Conclusions It may be concluded that L-arginine has beneficial effect on brain injury in acute ischemic stage and high dose provides better effects.