The mean end-expiratory oxygen concentration in nine volunteers breathing 100 per cent oxygen from a B.L.B. mask was only 61 per cent; furthermore there were wide differences in the efficiency of this mask between subjects. An alternative system of oxygen administration is described, consisting of a pilot's mask and a demand valve. The mean end-expiratory oxygen concentration attained with this system was 83 per cent and individual variations were small. This mask is more comfortable to wear than the B.L.B. and the inspiratory and expiratory pressures are very low.This study was carried out at two atmospheres absolute in order to allow measurements of arterial oxygen tension to be made in some of these subjects. The arterial values obtained indicate that the arterial oxygen tension approaches closely the level predicted theoretically at this pressure. Hyperbaric oxygen therapy is therefore not impeded by the failure of the arterial blood to reach the expected oxygen tension provided that the oxygen administration system is efficient.