Continuous measurements of transcutaneous PO2 are of interest in pulmonary function tests by which dynamic processes of pulmonary gas exchange are examined. The aim of the present study is to clarify the question of whether the transition function of transcutaneous PO2 after a step change of inspiratory oxygen concentration could give information about pulmonary gas exchange. In 8 healthy volunteers and 31 patients with chronic obstructive pulmonary disease of various degrees the half-value mixing times of transcutaneous PO2 were correlated with parameters of respiratory mechanics and pulmonary gas exchange. In spite of a damping of rapid changes of arterial PO2 by the transcutaneous measurement, there were significant correlations of the mixing times with physiologic right-to-left shunt; with physiologic dead space, alveolar ventilation, and blood gases; and with the parameters of lung overinflation and airway obstruction. There was a significant correlation with the age of the subjects, too. This had to be attributed mainly to an increase in severity of lung function disturbances with advancing age. Aging of the skin seemed to have a minor influence. According to these results, the transition function of transcutaneous PO2 after a step change of inspiratory oxygen concentration is determined mainly by the alveolar ventilation and the ventilation-perfusion inhomogeneity. This paper describes a short test that only takes some minutes of oxygen breathing and that has nearly the same diagnostic value as the measurement of the half-value mixing time.