Adult male rats exposed to tetrahydrofuran vapour at 8.2 (200 p.p.m.), 41 (1,000 p.p.m.) or 82 mumol/l (2,000 p.p.m.) for 2 to 18 weeks, five days a week, 6 hrs daily, showed dose-dependent brain and perirenal fat solvent burden linearly correlated to each other. After two weeks of exposure, the body burden of tetrahydrofuran seems to decrease. This might have been caused by increased oxidative metabolism as enhanced 7-ethoxycoumarin O-deethylase activity was detected in liver and kidneys in the 2nd week and onwards. The exposure also caused inhibition of alcohol and formaldehyde dehydrogenase activities in liver at the highest dose. Biochemical effects in the cerebellum were not detected while gluteal muscle specimens showed increased succinate dehydrogenase activity in a dose-related manner. This points to effects on the energy metabolism. Muscle acetylcholine esterase activity was also increased showing possible effects on the myoneural junctions.