The effects of a single oral dose or chronic ingestion of ethanol on in vivo and in vitro metabolism of enflurane were studied in Fischer 344 rats. At various intervals after ethanol treatment, enflurane was administered ip and 1 h after enflurane administration fluoride and ethanol levels were measured in plasma and hepatic microsomes were prepared. The concentration of ethanol in plasma (±SE) was 0.138 · 0.035% at 9 h after the single dose of ethanol and decreased almost to control levels by 17 h. The hepatic microsomal defluorination of enflurane was enhanced 3.5-fold and 6.3-fold at 9 and 25 h after the ethanol dose and returned to the control level by 33 h. In vivo defluorination was inhibited almost completely at 8 h after the ethanol dose, increased to 3.4 times the control level at 24 h, and decreased to the control level by 32 h. At 1 h after the end of chronic ethanol treatment, the concentration of ethanol in plasma was 0.254 · 0.018%, and it decreased to the control level by 9 h. Hepatic microsomal enflurane defluorinating activity was increased 10.5-fold at 1 h after the end of chronic treatment and decreased to the control level by 13 h. Immediately following chronic treatment, enflurane defluorination in vivo was almost totally inhibited. It increased to 9.3 times the control level at 4 h after chronic treatment was stopped and then decreased to nearly the control level at 12 h. Changes in hepatic microsomal enflurane defluorinating activity following ethanol administration were paralleled by changes in the amount of a microsomal protein that electrophoresed in the molecular weight region of the cytochrome P-450 isozymes. The results show that ethanol can produce a very rapid increase in hepatic microsomal enflurane defluorinating activity in rats and provide evidence that this increase results from the induction of a form of cytochrome P-450. Defluorination of enflurane in vivo can be inhibited or stimulated following treatment with ethanol, depending on the level of ethanol present when the enflurane is administered.