After trichloroethylene (Tri) inhalation the metabolite trichloroethanol (TCE) is excreted rapidly in the urine, while trichloroacetic acid (TCA) accumulates in the organism due to a high plasma protein binding rate. After exposure of 5 volunteers to 50 ppm Tri for 6 h, 50 μg/ml TCA is found in the plasma at the end of the 5th exposure day, the amount of TCE present in the blood amounting to not more than 2.3 μg/ml. Instead of the previously used control of TCE + TCA in the urine, it is suggested to determine the metabolites in the blood by means of a gas Chromatographic micro technique which provides a reliable criterion of the preceding Tri exposure.