In eight-hour inhalation experiments with volunteers in a gas chamber, the biotransformation of ethyl benzene, styrene, and -methylstyrene was investigated. About two-thirds of the vapors are retained in the respiratory tract. After the inhalation was terminated, only traces were found in the expired air in the case of ethyl benzene, but no exhaled styrene could be detected. The amounts of these hydrocarbons excreted in urine were negligible. The main urine metabolite of ethyl benzene and styrene in man is mandelic acid; a smaller portion is eliminated as phenylglyoxylic acid and, in the case of ethyl benzene, as methylphenylcarbinol. Mandelic acid is excreted rapidly from the organism; its determination in urine may be recommended as an index of exposure for ethyl benzene as well as for styrene. In urine of subjects exposed to a-methylstyrene, atrolactic acid was found.