Using the canine thalamic infarction model, we have investigated the effects of 10 ml/kg of 20% mannitol on the hemodynamics of the ischemic brain. Mannitol was found to cause an increase in rCBF, but it is not marked in animals with severe ischemic foci. Increases were statistically significant in the animals with moderate ischemia. Increases in rCBF due to mannitol administration lasted for about 1 hour, as did the rise in serum osmolarity. The actual relationship between these two parameters, remains unclear.