The effects of acute ethanol intoxication were investigated in a rat model of unilateral middle cerebral artery occlusion. Groups of 5 to 8 male Sprague-Dawley rats were subjected to 4 hr of left middle cerebral artery occlusion. All groups were deprived of food overnight and were pretreated intraperitoneally with 5% dextrose solution (10 ml/kg), 20% ethyl alcohol in 5% dextrose solution (2 g/kg), or 30% ethyl alcohol in a 5% dextrose solution (3 g/kg) 1 hr before middle cerebral artery occlusion. Regional cerebral blood flow during ipsilateral occlusion was approximately 9.1 to 10% of baseline in all groups. The mean % brain water content in control, 2 g/kg ethanol-treated groups, and 3 g/kg ethanol-treated groups were: in the ischemic core--81.6, 81.2, and 82.4; intermediate zone--80.5, 80.6, and 81.7; and outer zone--79.7, 79.7, and 80.8, respectively. Brain Na+ and K+ content in the three groups was related to water content, but much greater with ethanol pretreatment. The water content of the intermediate zones in the 3 g/kg ethanol-treated animals was significantly greater than in the control (p < 0.01 and 0.001) and the 2 g/kg ethanol-treated groups. One-way analysis of variance indicated a significant dose-effect relationship in which the lower dose of ethanol tended to reduce ischemic core water content, and the larger dose increased ischemic core water, compared with the control. None of the overnight fasted groups had any significant hyperglycemia. The group given 3 g/kg i.p. ethanol 1 hr before had exacerbated edema formation with a mean whole blood level of ethanol of approximately 230 mg/dl. The neurotoxic effects of high concentrations of ethanol were unrelated to any change in plasma glucose concentrations.