An analyzer for measuring the concentrations of volatile organic anaesthetic agents in inspiratory and end-tidal gases has been constructed. Respiratory gas from an anaesthetized patient is led continuously through a heated capillary transport tube (length 5.7 m, I.D. 0.25 mm) to a hydrogen flame-ionization detector. The pressure drop across the capillary tube necessary to transport the gas is applied by operating the detector at reduced pressure. The ionization current, caused by the organic anaesthetic agent in the detector, is measured with an electrometer amplifier. The transport time, at an optimal pressure drop of 600 mm Hg, is 4.3 sec, and the flow-rate of respiratory gas in the tube is then 3.9 ml/min. The time constant of the system is 0.2 sec. It is shown that mixing between successive inspiratory and expiratory samples can be neglected. The use of the system is demonstrated by two examples. Firstly, the end-tidal concentration of diethyl ether during the wash-out after a combined intravenous infusion-inhalation anaesthesia was measured. Secondly, the analyzer was used during experiments to measure the ventilation:perfusion ratio by administration of small concentration of halothane.