The appropriate administration of intravenous fluids in neurosurgical patients remains an area of disagreement between neurosurgeons and anaesthetists. Fluid restriction has long been advocated by the former and is widely believed to reduce or prevent the formation of cerebral oedema. However, such restriction can lead to hypovolaemia which in turn can result in haemodynamic instability. Thus, brain homeostasis should be aimed for through adequate fluid administration and normal or slightly elevated mean arterial pressure. The properties of the endothelium differ between the brain and the remainder of the body. In most non CNS tissues the size of the junctions between endothelial cells averages 65 A. Proteins do not cross these gaps while sodium does. In the brain, the junction size is only 7 A, which is too small to allow crossing by sodium. Investigations with changes in osmotic and oncotic pressure have demonstrated that: 1) reducing osmolality results in oedema formation in all tissues including normal brain; 2) a decrease in oncotic pressure is only associated with peripheral oedema but not in the brain; 3) in case of brain injury, a decrease in osmolality elicits oedema in the part of brain which remained normal; 4) similarly, a decrease in oncotic pressure does not cause an increase in brain oedema in the injured part of the brain. Thus, a major reduction in oncotic pressure is unimportant for the brain, whereas changes in total osmolality are the dominant driving force at this level. To conclude, in a hypovolaemic patient with severe head injury, the crystalloid of choice is NaCl 0.9% and the colloid of choice is hydroxyethylstarch, both with an osmolality > 300 mosm.kg-1. Ringer-lactate is hypoosmotic (255 mosm.kg-1) and may cause or increase cerebral oedema. Mean arterial pressure should be maintained above 80 mmHg.