The mean inhaled oxygen concentrations received from five devices commonly used for delivering supplemental oxygen to spontaneously breathing patients were determined. The ideal technic for the determination would required direct measurement of mean alveolar oxygen concentration. Since this is not feasible, we plotted arterial oxygen tensions against known inhaled oxygen concentrations in volunteers. The fitted equations were then used to determine mean inhaled oxygen concentrations received by the volunteers while breathing from each device. The volunteers received accurately determined oxygen concentrations from those devices employing the Venturi principle at settings of 40% or less. The concentrations received did not coincide with the settings with the Venturi devices above 40%, and were highly variable with the non-Venturi devices. Variability in the concentrations received was related to the fact that the volunteers' peak inspiratory flow rates exceeded total gas flows delivered by the devices.