Delivered enflurane concentrations from two calibrated Ethrane vaporizers were determined with total gas flows of 3,5 and 8 L/min. Regardless of total gas flow the presence of 60% nitrous oxide increased enflurane concentrations by 20 to 40% above those concentrations present when only oxygen was flowing through the vaporizer. This nitrous oxide effect was present at all dial settings studied except the lowest engraved (0.25) concentration. Enflurane output at the 0.25% setting was 0.38% with or without nitrous oxide. Maximum changes in enflurane concentrations after adding nitrous oxide required about 5 minutes but the rapidity with which enflurane concentrations approached this maximum value were directly related to total gas flow. Similar effects of nitrous oxide on enflurane output from Cyprane Ethrane vaporizers were also measured. The mechanism of increased vaporizer enflurane output in the presence of nitrous oxide is unknown but may reflect increased gas flow through the vaporizing chamber secondary to increase in gas density associated with nitrous oxide. A similar mechanism has been proposed to explain increased halothane concentrations delivered by Fluotec Mark 2 vaporizers in the presence of nitrous oxide. Clinically, central system stimulation and anesthetic overdose may occur from increased enflurane concentrations delivered when nitrous oxide is added to the gases flowing through the Ethrane vaporizer. The ability to deliver low enflurane concentrations is limited since the measured concentration at the lowest dial setting was nearly 0.4%.