Species, sex and individual differences in serum inorganic fluoride concentrations were demonstrated in mice, guineapigs and rats exposed to either 0.07% or 0.2% enflurane for 35 days, suggesting differences in enflurane biotransformation. Exposure of Fischer 344 rats and Sprague-Dawley rats to 0.2% enflurane for 8 days resulted in enzyme induction as demonstrated by increasing serum inorganic fluoride and cytochrome P-450 concentrations. However, there was no difference in cytochrome P-450 concentrations between the strains despite differences in inorganic fluoride concentration. These results emphasize the multiplicity of factors and the lack of predictability in patterns of enflurane metabolism among species, strains and individuals.