Animal studies of incapacitation by thermal decomposition products indicate that the common asphyxiant gases, CO, Ha, low 02 and C02 are almost certainly responsible for the severe narcosis and death of fire victims overcome by smoke. Eye and upper respiratory tract irritation also probably impair escape capability but to an unknown degree. Time to narcotic incapacitation in man should be predictable if the fire profile in terms of the above gases is known. Incapacitation of victims of smoldering and post-flashover fires can be explained in these terms, but victims should be able to escape from early flaming fires. It is suggested that the high incidence of victims in the room of fire origin may be partly due to sleeping victims being intoxicated by CO during the smoldering phase