After the administration of Dopamine to 20 patients after cardiac surgery pulmonary shunt volume was determined with regard to the cardiac output and pulmonary pressure. It was shown that the infusion of Dopamine leads to decrease of the arterio-venous oxygen difference (AVDO2) as well as to the decrease of the PO2 in the arterial blood. On the contrary, in venous bood, PO2, Oxygen saturation, content and pulmonary pressure increased. The increase of the pulmonary right-to-left shunt observed (Qs Qt form 14.9 to 19.2%) is in proportion with the increase in cardiac output. Hence we do not suppose a specific shunting effect of Dopamine. Our findings suggest, that in the case of an already reduced blood oxygen level a further fall of the arterial oxygen saturation caused by a shunt must be avoided by additional enrichment of oxygen content of the inspired air.