Development of synthetic resin industry is remarkable, and the works handling the material are increasing recently. An investingation on the relation between the amount of styrene gas inhaled and the excretion of metabolic substances into the urine was made by the determination of mandelic acid (M.A.) and phenylgyoxylic acid (P.G.A.) with respect to the consentration and the exposure time. The results obtained were as follows. 1) Regardless of the kinds of work engaged, the quantity of M.A. and P.G.A. can definitely be said to be proportional to the concentration of styrene gas inhaled and the length of exposure. 2) Even after the exposure to a comparatively low concentration of styrene gas ranging from 10 to 15 p.p.m. the excretion of M.A. and P.G.A. in the urine of subjects showed 2.7 to about 4 times higher than in control workers respectively. And in an environment with a high concentration, the amount of M.A. and P.G.A. excreted has reached 6 and 10 times respectively. 3) The M.A. value excreted into the urine of workers exposed to styrene gas was higher than that of P.G.A., and it is also disclosed that the M.A. value was also 4∼5 times higher than that of P.G.A. both in the morning and the afternoon in every workshop. 4) Workers in an enviroment with a high concentration of styrene gas could not recovered to the normal condition even in the next morning. This means that the workers who are showing still high concentrations of metabolites in the urine would again be exposed in the following day. 5) Even if the workers exposed to a high concentration of styrene gas have been transferred to a non-contaminated workshop, the amount of M.A. and P.G.A. in the urine did not show a rapid decrease. It took about 6 days until he recovered to the normal condition. 6) The increase of M.A. and P.G.A. in the urine of workers exposed styrene gas might be used as a kind of criterion for an occupational health measures, though the detected amount is not well correlated to the degree of poisoning.

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