The metabolic breakdown of halothane was quantiatively determined in two patients. Trifluoroacetic acid and bromide were found as metabolites in the urine. Both metabolites have a protracted excretion rate. Since the biological half-life of trifluoroacetic acid is unknown, one can calculate only the least amount of halothane that had been metabolized on the basis of the excreted trifluoroacetic acid: 12 per cent in both patients. On the basis of the excreted urinary bromide, 20 per cent and 17 per cent, respectively, of the halothane taken up by the body was calcalated to be metabolized, if one assumes a biological half-life of 12 days for bromide.