Tetrahydrofuran (THF), a four-carbon, cyclic ether, is widely used as an industrial solvent. Groups of 10 rats and mice of each sex were administered THF vapor by whole body inhalation for 13 weeks at exposure concentrations of 0, 66, 200, 600, 1800, and 5000 ppm. The body weights and survival were not affected by THF exposure, except in male mice at 5000 ppm concentration which had reduced weight and three deaths. Clinical signs of central nervous system (CNS) toxicity were observed in both rats and mice at high dose levels. Rats of both sexes exposed to 5000 ppm were ataxic and mice exposed to 1800 or 5000 ppm appeared to be in a state of narcosis. There were no exposure-related gross necropsy findings in rats or mice. At 5000 ppm, decreases in thymic and spleen weights in rats and mice of both sexes and increases in liver weights in both sexes of mice and in female rats were observed. A minimal to mild centrilobular hepatocytomegaly occurred in male and female mice exposed to 5000 ppm THF. Atrophy of the uterus and degeneration of the X zone in the adrenal cortex occurred in female mice exposed to 5000 ppm THF. THF exposure of rats was associated with minimal to mild acanthosis and inflammation in the forestomach. In conclusion, these studies suggest that THF, like other commonly used organic solvents, causes narcosis in rats and mice. Although minimal exposure-related effects were seen in the liver of both species, morphologic changes were present only in mice.(ABSTRACT TRUNCATED AT 250 WORDS)