The effects of mannitol infusion therapy on acute ischemic stroke were investigated by serial median nerve somatosensory evoked potentials (SEP) in 31 patients with abnormal SEP recordings within 72 hours after onset. In 10 patients with missing N20 waves mannitol had no effect. In 12 patients mannitol improved N20 and central conduction time (CCT) latencies. In 9 patients absent N20 waves appeared after mannitol. As regard to the location of infarcts, CT revealed 7 hemispheric infarcts in patients with absent N20 waves in spite of mannitol infusion. The remaining patients' CTs revealed cortical and subcortical infarcts. This study concludes that mannitol treatment improved microcirculatory flow in the ischemic penumbra of acute ischemic stroke patients. We suggest that SEPs can be used in evaluating the effects of drugs to be used in the therapy of acute cerebrovascular accidents.