Reversible opening of the blood-brain barrier before administration of watersoluble cytoreductive agents constitutes a new tool in the treatment of central nervous malignancies. In this study a volume of 172 ml mannitol 25% has been injected into one of the cerebral arteries within 30 seconds. 5 minutes later the osmolality had augmented from 286 to 303 mosmol/kg and the plasma volume by 15%. The hemodynamic measurements--cardiac index (+47%), heart rate (+7%), LVSWI (+50%), systemic vascular resistance (-20%) and blood volume (+15%)--indicate a biphasic response with early fall in SVR followed by a rise in cardiac performance which is independent of the preload.