The inspired oxygen fraction (FIO2) delivered by the Hudson Oxy-one face mask was measured under changing conditions of ventilation, oxygen flow rate to mask, and mask fit. A single trained subject sat in a body plethysmograph to measure ventilation and breathed at a constant rate of 15 per minute at three different tidal volumes, of approximately 0.3, 0.6, and 1.2 litres, from the mouthpiece in the plethysmograph. The Oxy-one face mask was fitted to a plaster-of-Paris face model on the outside of the plethysmograph in a loose and then in a tight fashion. Oxygen concentration was continuously monitored from a point in the metal tube connecting the face model to the mouthpiece. The tightly fitting mask demonstrated an orderly reduction in FIO2 as ventilation increased and oxygen flow rate to the mask decreased. The mean FIO2 at a ventilation of 4.5 l.min-1 and 8 l.min-1 oxygen flow was 78% and this fell to 27% at a ventilation of 16 l.min-1 and oxygen flow of 2 l.min-1. The loosely fitting mask demonstrated larger SD of measurements and lower mean maximum FIO2 values of 46 to 49% and these fell in an irregular fashion to similar minimum values as ventilation increased and oxygen flow decreased. Although the precise definition of the FIO2 for each breath from the changing concentration during each inspiration was not possible, these results indicate that FIO2 changes in a predictable way as a function of ventilation and oxygen flow, if the mask is close fitting. This method could be conveniently used to study other oxygen delivery systems.