1. The use of continuous infusion anaesthesia has only been of interest since the development of short-acting, less cumulative and less toxic drugs. 2. This study aimed to compare pharmacokinetics and haemodynamic effects during and after long time methohexitone constant rate infusion. Sixteen patients were given either 60 or 90 micrograms kg-1 min-1 methohexitone during 14 h. Blood samples were taken hourly during this time and 12 h following the end of infusion. 3. Infusion period was analysed by a single exponential model; post-infusion time showed a three compartment model, the intermediate phase parameters corresponding to those of the infusion period. 4. Methohexitone was haemodynamically well tolerated; prolonged infusion decreases oxygen consumption, mainly by a decrease in oxygen demand. 5. Many patients remained unconscious for unacceptably long periods of time after post-operative sedation by methohexitone.