The kinetics of biotransformation of methoxyflurane by rat hepatic microsomesin vitrowas studied. The rate or biotransformation as measured by analysis of metabolites continued to increase even at near-saturation concentrations of the anesthetic. Methoxyflurane biotransformation followed either an unbounded curve with empirical formula y = a ln(bx + I) or an asymptotic curve with formula y = x/ax+b No substrate inhibition was observed, Total fluoride Vmaxof 135.1 mμmol F-/mg protein/30‘ was increased to 931.9 by phenobarbital induction: free fluoride Vmaxfrom 39.2 to 403.2. Thus, enzyme induction shifted biotransformation to the production of greater amounts of inorganic free fluoride metabolites than organic fluoride-contaning metabolites, Phenobarbital induction caused qualitative as well as quantitative alteration in the biotransformation of methoxyflurane.