1. Rats killed in a variety of ways (broken neck, nembutal, anoxia, electrocution) may undergo extensive bubble formation when subsequently decompressed from atmospheric pressure to simulated altitudes of 50,000 feet. On autopsy at sea level, large numbers of bubbles are found throughout the vascular system in the majority of animals. These bubbles appear to originate in small vessels deep within muscular regions, later spreading widely in arterial and venous systems. Dead rabbits and frogs also bubble profusely on decompression. 2. Bubble formation in dead animals is attributed primarily to the accumulation of CO(2), derived from residual cellular respiration after death, and from anaerobic glycolysis with attendant decomposition of bicarbonates in blood and tissue fluids. If anaerobic glycolysis is inhibited by using sodium iodoacetate as a lethal agent, bubble formation is greatly reduced or lacking on subsequent decompression. 3. Experiments in vitro suggest that high concentrations of CO(2) favor bubble formation by reducing the degree of mechanical disturbance necessary. 4. Administration of CO(2) in high concentrations to living frogs lowers the minimum altitude (pressure equivalent) at which bubble formation occurs, with exercise, in untreated animals. Pre-treatment with CO(2) also reduces the degree of muscular activity necessary for bubbles to form in frogs at higher altitudes. 5. Analyses have been made of the gas content of bubbles taken directly from the large veins of decompressed frogs and rats. In living animals the figures obtained indicate rapid equilibration with gas tensions in the blood. Bubbles taken from decompressed dead rats may contain 60-80 per cent CO(2). 6. The bearing of these experiments on the mechanisms of bubble initiation and growth in normal living animals is discussed. Reasons are given for suggesting that CO(2), due largely to its high dissolved concentration in localized active regions, may be an outstanding factor in the initiation and early growth of bubbles which in later stages are expanded and maintained principally by nitrogen.