THE local function of different regions of the lungs can be studied by the inhalation of various gases labelled with oxygen-15. Recently, water vapour has been labelled in this way. Oxygen-15 has a half-life of 2 min. and is produced by a continuous flow process by bombarding nitrogen with deuterons in the Medical Research Council cyclotron in the grounds of Hammersmith Hospital. Details of its preparation have been published1. The oxygen-15 is available in nitrogen containing 3 per cent oxygen as carrier at a flow of 0.45 litre per minute. Radioactive water vapour was prepared by adding 4 per cent hydrogen in nitrogen at a flow of 1 litre per minute and passing the resulting mixture over a palladium-type catalyst (‘Catalyst D’; Baker Platinum Co.). The effluent contained 2 per cent of water as a vapour and had an activity of about 10 mc. per litre. The absence of unconverted oxygen was checked with a mass spectrometer.