The intravenous administration of Althesin 50 µ/kg to 9 patients undergoing neuro-surgery during general anaesthesia with controlled ventilation resulted in a fall in intra-cranial pressure (i.c.p.) measured in one lateral ventricle. The extent of the reduction in intracranial pressure was directly proportional to the starting value of pressure. Intra-cranial pressure gradually returned to control values over 10 min. Blood pressure also fell significantly but the extent of its reduction was not sufficient in itself to account for the change in intracranial pressure. It is postulated that the intracranial pressure fall was the result of a reduction in cerebral blood flow and cerebral blood volume. Althesin would therefore appear to be a satisfactory drug for use in patients with intracranial compression.