Excessive calcium influx has been implicated in the pathophysiology of ischemic cerebral damage. The effects of nimodipine, a calcium antagonist on the Na+-K+/MG(+2) ATPase activity, Ca+2/Mg+2 ATPase, lipid peroxidation, and early ultrastructural findings were examined at the acute stage of ischemia in the rat brain. Ischemia was produced by permanent unilateral occlusion of the middle cerebral artery. In Group I; the rats which had no ischemia and not received medication were used for determining Na+ -K+/Mg+2 ATPase, Ca+2/Mg+2 ATPase, the extent of lipid peroxidation by measuring the malondialdehyde content and normal ultrastructural findings. In Group II, the rats which had only subtemporal craniectomy without occlusion and received saline solution were used for determining the effect of the surgical procedure on the biochemical indices and ultrastructural findings. ln Group iii, the rats received saline solution following the occlusion in the same amount of nimodipine and in the same duration as used in Group IV. In Group IV, nimodipine pre-treatment 15 min before occlusion (mug kg(-1) min(-1) over a 10 min period) was applied i.v. Na+-K+/Mg+2 ATPase and Ca+2/Mg+2 ATPase activities decreased significantly and promptly as early as 70min and remained at a lower level than the contralateral hemisphere in the same group and at the normal level in Group 1. Nimodipine pre-treatment immediately attenuated the inactivation of Na+ -K+/ Mg+2 ATPase (p < 0.05) but there was no change on Ca+2/Mg+2 ATPase activity (p < 0.051. Malondialdehyde content increased significantly in Group III following ischemia as early as 30 min. Nimodipine pre-treatment decreased the malondialdehyde level in Group IV (p < 0.05). This study supports the possibility that nimodipine pre-treatment effects the membrane stabilizing properties via inhibiting the lipid peroxidation and subsequently restoring some membrane bound and lipid dependent enzymes' activity such as Na+-K+/Mg+2 ATPase and the ultrastructural findings.