It was previously reported that the elevation of inorganic fluorine level in plasma after inkalation of methoxyflurane causes acute renal failure. This study investigated whether the peak concentration or the duration of the inorganic fluorine is responsible for the real failure. Continuous infusion (5 millimols/liter and 10 millimols/liter) or a single intravenous bolus injection (720 millimols/liter) of sodium fluoride solution was adminstered to rabbits. The plasma concentration and the duration of the plasma fluorine was measured. There were no signs of pathological or biochemical changes that suggested renal failure when the peak plasma concentration was less than 50 millimols/liter for over 4 hours (5 millimols/liter, 24 hours), or whether peak concentration of plasma inorganic fliorine was over 50 millimols/liter for less than 4 hours (720 millimols/liter, intravenous bolus). However when 10 millimols/liter of sodium fluoride solution was administered at a speed of 10 milliliters/hour for 24 hours, the rabbits showed a peak plasma inorganic fluorine concentration over 50 micromols/liter for more than 4 hours and signs of renal failure developed ata plasma concentration of 65.8 micromols/liter 24 hours after the beginning of infusion. renal failure was mainly inthe form of edema of the tubular cells in both the cortex and medulla and abnormal biochemical changes (blood urea nitrogen 55.8±12.5 milligrams/deciliter, plasma creatinine 1.2±0.1 milligrams/deciliter). Inorganic fluorine can cause renal changes when its paek plasma concentration reaches more than 50 micromols/liter and lasts for more than 4 hours.