A possible role of Ca2+ -activated potassium efflux in brain ischemia was studied using a rat focal cortical infarction model. Three days after ischemic insult, tissue contents of water, sodium, potassium and calcium ions were measured. Charybdotoxin, a specific inhibitor of Ca2+ -activated potassium efflux, was found to reduce the formation of ischemic brain edema when a dosage of 0.15 mg/kg was given by i.v. 20-30 min prior to the onset of ischemic insult.