We have assessed the performance of the Hudson-type mask without reservoir and the Hudson non-rebreathing mask with reservoir bag in 10 pregnant and 10 non-pregnant women at different oxygen flow rates by measuring end-expired oxygen fraction (FEO(2)). The Hudson mask without reservoir behaved similarly in pregnant and non-pregnant women, providing a mean FEO(2) of 0.32 with oxygen flow of 4 L.min(-1) which increased to 0.46 at 15 L.min(-1). The reservoir mask produced a lower FEO(2) in pregnant subjects than in non-pregnant subjects, with a mean FEO(2) of 0.33 and 0.64 at 4 L.min(-1) and 15 L.min(-1) in pregnant subjects compared to 0.44 and 0.72, respectively, in non-pregnant women. Neither mask delivers the expected maximum inspired oxygen fraction quoted by its manufacturer, assuming that inspired oxygen fraction quoted by its manufacturer, assuming that inspired oxygen fraction is 0.06 higher than expired and correcting for humidification of inhaled gases within the respiratory tract. The importance of these findings in the care of the obstetric patient and the treatment of acute fetal hypoxia is discussed.