Oxidized irwN has been proposed as a mediator of the free radical-induced damage that occurs during cerebral ischemia. Dihydroriboflavin, a compound produced from riboflavin (B2) by NADPH-dependent flavin reductase, rapidly reduces oxidized iron. Since treatment with riboflavin offers protection from ischemic injury in other tissues, we tested the effect of pretreatment with B2 on brain edema formation during focal ischemia. Two different models of middle cerebral artery occlusion (MCAO) in rats were tested: transcranial electrocautery and intracarotid occlusion with a nylon thread. Groups of 6-8 animals were treated with 7.5 mg of B2/kg or saline vehicle 1 h before MCAO and brain water content was determined after 4 h of ischemia. Pretreatment with B2 reduced total hemisphere edema formation from 0.37 +/- 0.05 to 0.19 +/- 0.05 mg/g dry wt. (48% protection, p < 0.01) following transcranial MCAO. Edema was greater following MCAO with the intra-carotid thread (0.54 +/- 0.05 ml/g) but protection by B2 was less (21%). We conclude that pretreatment with B2 reduces ischemic brain injury, perhaps by reacting with oxidized iron. However, the larger stroke produced by the thread MCAO method makes it more difficult to observe protection following brief ischemia in this model.