Two techniques of pre-oxygenation were studied by continuous analysis of respired gases using a mass spectrometer in 10 healthy volunteers. The first was a conventional technique as commonly used in anaesthesia with a Bain system and tightly-fitting anaesthetic facemask and an oxygen flow of 8 l.min-1. The second technique also used a Bain system with an oxygen flow of -8 l.min-1, but with a Hudson mask attached. The mean fractional end-tidal oxygen concentrations after 3 min were 0.812 and 0.46 respectively for each of the pre-oxygenation techniques against 0.16 for subjects breathing air. This represents a considerable increase in pulmonary oxygen reserve for both techniques. The second technique is not an alternative to conventional pre-oxygenation for emergency anaesthesia, but is a useful and simple method that is acceptable to both patient and anaesthetist in routine cases.