Changes in cerebrospinal fluid pressure, venous pressure, and arterial pressure induced by different concentrations of halothane were measured in ten elderly patients prior to the beginning of surgery. Cerebrospinal fluid and venous pressures were recorded by a water manometer and arterial pressure with a sphygmomanometer. Tidal volume, pH, and pCO2 were determined intermittently in order to prevent changes in respiratory physiology. The patients were intubated with a cuffed endotracheal tube and given 1/2 to 2 vol. per cent halothane, for periods of 15 minutes or longer, using a non-rebreathing system and a Fluotec vaporizer.