A method is described for the estimation of carbon dioxide output, oxygen uptake, and respiratory exchange ratio during nitrous oxide anaesthesia. The inspired and expired minute volumes are measured with a box-bag system, the carbon dioxide concentrations with a carbon dioxide sensitive electrode, and the oxygen concentrations with a polaro-graph. Carbon dioxide output can be measured with a coefficient of variation of 1.1 per cent. The error in the measured oxygen uptake is largely dependent upon the total inspired oxygen but the error has a standard deviation of the order of 13 ml. Applications of the technique are discussed.