In experiments in dogs on the metabolic effects of inhalation anaesthetics, we noticed that in the presence of desflurane, oxygen uptake (VO2) measured with the Deltatracll metabolic monitor seemingly increased whereas it decreased when determined independently by the Fick principle. This difference remained even after correction for changes in gas concentration on addition of an inhalation anaesthetic. Therefore, we suspected that desflurane interferes with the measurement of gas concentrations. Using different precision gases, we found that desflurane disturbed both the paramagnetic oxygen sensor and the infrared carbon dioxide detector so that the measured oxygen (when FIO2 was > 0.21) and carbon dioxide concentrations were greater than expected. These errors multiply in the computing process of oxygen uptake by the DeltatracII. When the DeltatracII is to be used during inhalation anaesthesia, its results should be corrected for the presence of an anaesthetic gas. More importantly, corrections must also be made for measurement errors of the oxygen and carbon dioxide sensors, unless the device has been equipped with a modified (nickel membrane) oxygen sensor insensitive to the presence of volatile agents.