The estimation of oxygen consumption and carbon dioxide elimination is essential for predicting the metabolic activity and needs of any patient having anaesthesia. During anaesthesia oxygen consumption can be measured and compared to a predicted value. However, oxygen uptake is affected by anaesthetic agents, which complicates the interpretation of measured oxygen uptake rate. The purpose of this study was to investigate whether there are any differences in respiratory gas exchange during anaesthesia with enflurane and isoflurane and also to assess the effects of spontaneous versus controlled ventilation. METHODS. Forty orthopedic patients were randomized to enflurane or isoflurane anaesthesia in nitrous oxide with either spontaneous or controlled ventilation. A fresh low-gas-flow technique was used. Inspiratory oxygen and end-tidal carbon dioxide concentrations and expiratory minute ventilation were measured in a circle absorber system between the y-piece and the endotracheal tube with a sampling analyser. Between the mixing box and the absorption canister, carbon dioxide concentration was continuously measured. The carbon dioxide elimination was calculated from mixed expired concentration and expiratory minute ventilation. Excess gas was collected every 10 min in a non-permeable mylar plastic bag connected to the excess valve. The excess gas flow was calculated and the oxygen uptake rate was assumed to be the difference between the oxygen fresh gas flow and the oxygen excess gas flow. RESULTS. The grand mean oxygen uptake rate was 2.5 ml.kg-1 x min-1 or 100 ml.min-1 x m-2. There were no statistically significant differences in oxygen uptake between enflurane and isoflurane anaesthesia or between spontaneous and controlled ventilation. The mean oxygen uptake rate at 10 min was between 2.0 and 2.2 ml.kg-1 x min-1 in all groups. At 30 min the mean oxygen uptake rates were 2.6 to 2.8 ml.kg-1 x min-1. Carbon dioxide elimination was closely associated with expired minute ventilation, with a carbon dioxide excretion of about 30 ml per litre gas exhaled, irrespective of ventilatory mode employed.