The mode of anaesthesia used during pneumoencephalography has a significant effect on the size of the cerebral ventricles 24 hours after the procedure. Post-encephalographic ventricular enlargement is less marked in patients examined under nitrous oxide anaesthesia. This appears to be related to passage of the gas into the ventricles during the encephalogram, and subsequent diffusion outwards. Variations in arterial carbon dioxide tension during the anaesthesia do not contribute significantly to changes in ventricular size. However, both hyperventilation and inhalation of nitrous oxide may cause apparent increase in size of the cerebral sulci.