The measurement of hypopharyngeal FIO2, is a simple and rapid method for evaluating oxygen delivery systems in clinical practice. Hypopharyngeal FIO2 measurements were compared to other established methods of inspired gas measurement, namely, tracheal gas sampling and exhaled gas analysis. Hypopharyngeal sampling and exhaled gas analysis. Hypopharyngeal sampling compared favourably with these methods. Furthermore, arterial blood gas samples from patients receiving oxygen by aerosol mask correlated with the hypopharyngeal measurements but not with the delivery system's settings. In normal, healthy subjects, the authors studied variables known to influence FIO2. Using nasal prongs as well as an aerosol mask system, the authors studied the interaction of the wall flow rates, the concentration (Venturi) settings (in the case of the aerosol mask), and the subject's respiratory rate. Under these controlled conditions of tidal breathing, the hypopharyngeal FIO2 was directly related to the total flow of gas from the delivery system and the concentration of the Venturi setting but inversely related to the respiratory rate. Hypopharyngeal gas sampling offers a simple method for evaluating and correcting a patient's inspired oxygen concentration.