The aspiration of 800 ml of fresh water and of sea water into one lung at a hydrostatic pressure of 10–15 cm was studied in dogs. The experimental model permitted normal oxygenation and normocapnia by isolated ventilation of the other lung. This was demonstrated by arterial blood gas analysis. Sodium, potassium, chloride and hematocrit values were monitored in the pulmonary vein blood of the flooded lung and in the left ventricle. Marked changes were found in the pulmonary vein blood, but virtually no changes were recorded in the blood from the left ventricle. In the fresh-water cases, slight myocardial damage was detected in the electrocardiogram. The sea-water cases showed no substantial changes in the electrocardiogram. It was clear that both fresh and sea water entered the circulation but only to a limited amount due to disturbance of the perfusion.