The present study was designed to test the effectiveness of furosemide in reducing cerebral edema due to closed head trauma. A Remington Humane Stunner was used to deliver blows to the heads of anesthetized cats. Impacted animals were divided into three groups: (a) trauma, no drug, and ad lib. fluid intake after head injury; (b) trauma, no drug, and standardized fluid intake (0.9% NaCl; 10 ml/Ib/day); and (c) trauma, furosemide (3 mg/lb/day), and standardized fluid intake. For the treated cats, we began furosemide therapy 1 hour after head injury and used three intramuscular injections/ day. The animals were killed 48 hours after head trauma. From animals with unilateral contusion, we took bilateral white matter samples from five points along the centrum semiovale and tested for edema using density determinations with an organic density gradient. Serum electrolytes, blood urea nitrogen, and weight were determined before the cats were impacted and before they were killed. All impacted animals demonstrated weight loss. Density data showed a normal water content for white matter in the uncontused hemispheres of all impacted cats. Contused hemispheres showed a significant decrease in density (increase in brain water content) for both treated and untreated cats. In the furosemidetreated animals, however, the brain edema was significantly less than that found in the untreated groups. Analysis of data from individual brain sections of furosemide-treated animals suggested a reduction in the spread of edema fluid 48 hours after head injury.