Cytochrome C prepared from beef hearts was stable, non-toxic in large doses, recoverable from the blood, and in increased amts. from the organs after intravenous and intramuscular injns. and apparently broken down before excretion because it was not recoverable in the urine. Following the intraven. injn. of cytochrome C, there was considerable increase in the content of this substance in the heart, liver, and kidney of the rat and rabbit. There was a similar, though delayed, increase following intramusc. injn. It was demonstrated by in vitro expts. that an increase of cytochrome C, similar to that produced in living organs by injn. was sufficient to produce a significant increase in tissue O2 uptake. By daily intraven. injns., a satisfactory blood level was maintained. Under conditions of anoxia, the liver content of cytochrome C increased after injn., whereas the blood level decreased. With release from anoxia, there was a decrease in liver content and an increase in blood level.