Recent measurements of cardiac output have shown that, during anaesthesia, departures from normality are of common occurrence. In the presence of pulmonary venous admixture, such changes of cardiac output may affect the arterial Po2 to a considerable extent. This paper explores the nature of the theoretical relationship between cardiac output, percentage pulmonary venous admixture, and (A-a) Po2 difference, and from the results draws certain conclusions: (1) Appreciable reductions in arterial Po2 during and after anaesthesia may be caused largely by reductions of cardiac output. (2) When allowance is made for the probable changes of cardiac output, it appears that the percentage pulmonary venous admixture during anaesthesia may not be greatly increased above the normal range. (3) Since with certain anaesthetic techniques there is a linear relationship between Paco2 and cardiac output, an increase of alveolar ventilation may under such circumstances cause a paradoxical decrease of the arterial Po2.