Absorption of m-xylene and excretion of m-methylhippuric acid were investigated under controlled conditions in ten volunteers aged 17-33 years. They were exposed to m-xylene vapours at concentrations of 100, 300, and 600 mg/m3. It was found that m-xylene vapour retained in the lungs tended to decrease at the end of the exposure. An exposure test was devised, based upon the results obtained during our investigations. The amount of xylene absorbed in the course of eight hours' work can be calculated with an accuracy of about +/- 8%.