To investigate the sodium magnetic resonance (MR) imaging characteristics of acute vasogenic edema, an experimental canine model was developed. Vasogenic edema was produced in the hemisphere of the dogs by the intraarterial infusion of hypertonic mannitol (25%). This solution opens the blood-brain barrier, allowing the influx of water, electrolytes, and proteins into the brain. The main advantage of this model over the established "cold injury" model is the lack of associated brain necrosis. Two patients with chronic vasogenic edema secondary to well-circumscribed meningiomas also underwent MR imaging. The sodium signal was markedly elevated in both clinical and experimental studies of vasogenic edema fluid compared with signal in healthy brain tissue. Extracellular sodium associated with vasogenic edema displayed MR imaging characteristics similar to that of sodium in serum. There was a trend toward a shortened T2 in edema fluid secondary to the presence of serum macromolecules.