To determine the urinary and biliary excretion of metabolites of halothane in dogs, 12 beagles were anesthetized with halothane either at 0.5 MAC (minimum alveolar concentration) for 1 hr or at 1.4 MAC for 4 hr. Urine and bile were then collected for 11 days following the anesthesia. The concentrations of inorganic fluoride in the urine and bile were measured with a fluoride electrode and an ion meter. The concentration of total fluoride containing organic fluoride also was measured in the same manner after conversion of the organic fluoride to an inorganic form by combustion. The concentration of trifluoroacetic acid (TFA) in the urine was measured by ion chromatography and that in the bile by gas chromatography. Over 80% of all the fluoride was excreted in the urine as organic fluoride in both groups. While the fraction of TFA in the organic fluoride in the bile was about 30% in both groups, that in the urine was 40% in the 0.5 MAC group and 65% in the 1.4 MAC group. Therefore, it was concluded that the organic fluoride compounds, the metabolites of halothane, and in particular TFA, were excreted mostly into the urine. The extent of metabolism of halothane decreased from 7.6% in the 0.5 MAC group to 4.9% in the 1.4 MAC group. The urinary excretion rate of TFA, however, was not affected by the concentration of inspired halothane.