The effects of pre-oxygenation were studied by continuous expired oxygen analysis in twenty pregnant patients and ten nonpregnant controls. The Magill system and a demand valve breathing system were both studied with tidal and vital capacity breathing. Rapid pre-oxygenation was possible with the demand valve system but the technique was associated with air leaks around the mask. Vital capacity breathing with the Magill system was inefficient in practice because of rebreathing and air leaks, and the recently proposed four vital capacity breath technique provided suboptimal alveolar oxygenation in pregnant subjects. Oxygenation in pregnant subjects was significantly faster than in nonpregnant controls and was achieved after just 2 minutes of tidal breathing from a standard Magill system with a prefilled reservoir. Care was necessary to ensure a gas-tight seal around the facemask.