We evaluated the sensitivity of magnetic resonance (MR) imaging in documenting effects of nimodipine in experimental focal cerebral ischemia. Twenty-five Sprague Dawley rats underwent unilateral occlusion of the middle cerebral artery and were imaged at different intervals thereafter. Neuropathologic and neurologic data were correlated with MR imaging results. Compared with controls, nimodipine-treated rats showed a significantly smaller infarct size (p less than 0.001), as documented by MR imaging and confirmed by neuropathologic evaluations. A less intense signal on the T2 weighted sequence was found in nimodipine-treated rats in basal ganglia (p less than 0.001) and cortex (p less than 0.05). MR imaging may afford unprecedented diagnostic sensitivity in assessing pharmacologic efficacy in cerebral ischemia.