From the practical standpoint of view the A-aDO2 proved to be a good parameter for evaluation of global gas exchange. The endexspiratory oxygen pressure, recorded by mass spectrometry, is set equal to the alveolar oxygen pressure. Therefore it is possible for clinical use to have an on line monitoring of PETO2--PaO2. It is necessary to specify the pressure difference in dependence of the inspiratory oxygen pressure since the alveolo-arterial oxygen pressure difference increases with increasing inspiratory oxygen pressure concentration. For practical clinical purposes we come out with the relative ratio of the difference adjusted to the alveolar oxygen pressure. The alveolar-arterial oxygen quotient also called "quotient": formula: (see text) has proved to be a clinical suitable figure. For this quotient are no standard norms reported in the literature. This study was set up to evaluate the quotient in 9 healthy volunteers. The standard norms, found in our study, can basically used for practical clinical purposes. This quotient is demonstrated to be practicable in intensive care medicine (obstructive pulmonary disease, acute pulmonary failure, positive pressure ventilation).