Ventilated volumes, arterial oxygen content and carbon dioxide tension, cardiac output, ‘effective’ pulmonary capillary flow, systemic arterial, pulmonary arterial and pulmonary arterial wedge pressures were determined before and after the intratracheal administration of 1–3 ml/kg body weight of fresh and sea water in 23 intact and 14 vagotomized sheep in light thiopentone anesthesia. Five untreated animals served as controls. A precipitous fall and a prolonged depression of arterial oxygen saturation was observed, largely independent of the amount of fluid inhaled. Hypoxemia was due to a gross increase in venous admixture resulting from the perfusion of large unventilated portions of the lungs. Fluid aspiration was also followed by a significant degree of pulmonary hypertension caused by arteriolar constriction in the lungs. Pulmonary hypertension was found to be unrelated to hypoxia. Submitted on October 15, 1959