Because oxygen and carbon dioxide are very soluble in certain silicone oils and fluorocarbon liquids, these liquids will support respiration of mammals. Mice and cats respiring silicone oil die shortly after return to air breathing, while those breathing fluorocarbon survive for weeks. The respiration of mice is optimally supported by these organic liquids at about 20 degrees C. In cats, arterial oxygenation is excellent, but there is some impairment of carbon dioxide elimination. All animals have suffered some pulmonary damage from breathing fluorocarbon liquids. Continued investigation of organic fluid respiration may lead to development of a safe method to support the respiration of man by liquids equilibrated with gases at atmospheric pressure.