We measured in 8 experimental studies the oxygen and carbon dioxide partial pressure in systemic arterial, mixed venous and wedge-blood. The results were: 1. In healthy mongrel dogs with controlled intermittent and continuous positive pressure ventilation (FIO2 0,21 and 1,0) paO2 greater than pwO2 greater than pvO2 and pwCO2 less than paCO2 less than pvCO2. 2. During experimentally produced left-sided atelectasis of the lung and positioning of the catheter in the pulmonary artery a) in the ventilated lung we found pwO2 greater than paO2 greater than pvO2 and pwCO2 less than paCO2 less than pvCO2 b) in the atelectatic lung we found paO2 greater than pwO2 greater than pvO2 and paCO2 less than pwCO2 less than pvCO2. According to these findings the sample of highly saturated mixed venous blood from a catheter in the pulmonary artery should be a hint that pulmonary capillary blood is mixed with central venous blood.