The biotransformation of Ēthrane was studied in seven healthy female patients by measuring urinary fluorine excretion. The total amount of Ēthrane recovered was 85.1 per cent of the amount absorbed; 82.7 ± 18.8 per cent was recovered as unchanged Ēthrane in exhaled air and 2.4 per cent as nonvolatile fluorinated metabolites in urine. Of the urinary fluorine, 0.5 per cent was excreted in inorganic form and 1.9 per cent in organic form. Following anesthesia, urinary excretion rates of fluoride in reached a maximum in seven hours. Maximum excretion of organic fluorine metabolites was reached on the second day. Urinary excretion then assumed a simple exponential decay, with half-times of 1.53 days for inorganic fluoride and 3.69 days for or-ganic fluorine. The excretion of unaltered Ēthrane in exhaled air assumed a three-term exponential decay, with half-times of 17.8 minutes, 3.2 hours, and 36.2 hours.