Urinary and faecal excretion of radioactivity after either an intravenous or oral (1 mg/kg) dose of 35S-labelled Tobias acid (2-naphthylamino, 1-sulphonic acid), a dyestuff intermediate structurally similar to the powerful carcinogen 2-naphthylamine, was studied in rats. The Tobias acid was eliminated from the body within 24 hours of administration, almost exclusively through the urine. TLC-chromatography of faecal extracts and urine did not disclose the presence of excreted products other than unchanged Tobias acid and the search for inorganic 35SO4 in the urine by BaCl2 precipitation was negative. There was significant absorption from the gastrointestinal tract, but neither cleavage of the sulphonic group nor other biotransformation by the intestinal flora was apparent under the test condition. There was no evidence that the sulphonic group of Tobias acid is cleaved in the body to a significant extent to give 2-naphthylamine. This information should help in the evaluation of the occupational hazard potential of Tobias acid.