SUMMARY Changes in cerebrospinal fluid pressure after induction of anaesthesia with Althesin were recorded in 21 volunteer patients who had no abnormality of the central nervous or cardiorespiratory systems. In patients who breathed spontaneously, cerebrospinal fluid pressure fell by 45% at 1 min after intravenous injection of Althesin 0.1 ml/kg. It returned almost to preinduction levels by 12 min During this time, there was a slight increase in Pa co 2 . When Pa co 2 was controlled by mechanical ventilation, the cerebrospinal fluid pressure fell slightly more; by 53% when Pa co 2 , was kept between 38–42 mm Hg and by 50% when Pa co 2 was maintained between 30–34 mm Hg. The significant fall of cerebrospinal fluid pressure produced by Althesin in conjunction with the rapid clear-headed recovery, may allow Althesin to be a valuable anaesthetic for neurosurgical procedures.