Mannitol, the osmotic diuretic used in neuroanaesthesia and neurointensive care, has, in addition to its osmotic properties, various effects upon haemodynamics, cerebral blood flow and cerebral blood volume. Three factors are proposed to contribute to mannitol's capacity to lower intracranial pressure and to improve cerebral compliance: cerebral dehydration, and two forms of autoregulation-mediated vasoconstriction. In the case of viscosity autoregulation, it is admitted that changes in blood viscosity after mannitol result in reflex vasoconstriction to maintain cerebral blood flow constant. It has also been proposed that when mannitol administration results in increased cerebral perfusion pressure, vasoconstriction may occur in vascular beds in which autoregulation to perfusion pressure is preserved. On the basis of its effects on cerebral blood flow and free radical scavenging properties, mannitol has recently been investigated as a cerebral protective agent, with the capacity to reduce or prevent damage due to cerebral ischaemia. Finally, mannitol may be injected into a carotid or a vertebral artery to produce blood-brain barrier breakdown, thus improving the brain penetration of chemotherapeutic agents.