Acid-base parameters and oxygen tension were studied in arterial blood and lumbar CSF of 45 subjects in three groups of controls, head injuries, and postoperative cases. CSF in both groups of head injury and postoperative cases showed changes of metabolic acidosis with the arterial blood changes of respiratory alkalosis. Both groups had significant hypoxaemia. CSF acidosis is an indication of cerebral metabolic disturbance. It is proportional to the severity of brain damage, and permits a prognosis for further clinical progress. It is recommended that patients with severe brain lesions should be put on controlled ventilation to relieve them of respiratory overwork and to combat cerebral oedema.