Totally closed circuit anaesthesia has been used with nitrous oxide/oxygen gas mixtures, supplemented by halothane for spontaneous breathing, or by opiates for artificial ventilation. Oxygen expenditure averaged 227 ml/min, while nitrous oxide expenditure declined exponentially from an initial value of 462 ml/min to 110 ml/min after 2 hr. These flow rated included gas sampled for analysis. Inspired oxygen concentration was maintained at 30 per cent by monitoring with a paramagnetic analyser. Halothane expenditure average 3.5 ml (liquid) per hr. Halothane concentrations in the operating theatre atmosphere did not increase above 0.03 p.p.m.