Methoxyflurane is capable of producing high-output renal failure in some patients and animal models, probably through metabolic liberation of free fluoride. The tubular site of action of fluoride was examined in Fischer 344 rats using clearance techniques. Free water reabsorption (TCH2O) and free water excretion (CH2O) were measured during mannitol or water diuresis in control rats and in rats given methoxyflurane or pretreated with sodium fluoride. Pretreatment produced statistically significant increases in urinary flow (from 10.5 +/- 1.4 to 20.1 +/- 1.9 mul/min/100 g b. wt.), in glomerular filtration rate (from 814 +/- 31 to 1,039 +/- 53 mul/min/100 g b. wt.), in per cent sodium excretion (from 0.107 +/- 0.008 to 0.155 +/- 0.015 per cent), and in per cent water excretion (from 1.27 +/- 0.15 to 2.00 +/- 0.20 per cent). Free water excretion remained relatively unaltered in rats pretreated with fluoride, perhaps due to elevated glomerular filtration rate and/or reduced proximal tubular reabsorption combined with inhibition of reabsorption in the ascending loop. Percentage free water reabsorption, on the other hand, was markedly reduced by the pretreatment, from 2.66 +/- 0.21 to 0.66 +/- 0.09 per cent. The observations are consistent with the hypothesis that fluoride inhibits tubular reabsorption primarily in the medullary portion of the ascending limb of Henle’s loop, perhaps by inhibition of an active chloride pump located in this nephron segment.