Fluoride balances were measured in man under strictly controlled dietary conditions before and during the intake of sodium fluoride, and after its discontinuation. Fluoride intake averaged 4.4 mg/day in the control period and 13.8 mg during the addition of an average of 21 mg sodium fluoride/day. The main pathway of fluoride excretion was via the kidney. Urinary fluoride excretion corresponded to about 50 per cent of the fluoride intake in the control study and during the intake of sodium fluoride. Fecal fluoride excretion was very low, averaging 0.29 mg/day in the control period and 0.88 mg/day during the sodium fluoride study. Fluoride balances were positive in all study phases and averaged +1.9 mg/day in the control period and +5.4 mg/day during the addition of 9.1 mg fluoride/day given as sodium fluoride. Fluoride retention corresponded to 42.9 per cent of the fluoride intake in the control period and to 39.3 per cent in the sodium fluoride study. Following the discontinuation of sodium fluoride, the fluoride balances were lower than in the control study. The loss of fluoride in sweat was not determined in these balance studies.