The effect of nitrous oxide on the elimination of halothane was studied in 10 patients ranging in age from 20 to 50 years. After establishing a stable baseline (inspired halothane concentration: 0.85%, end-tidal halothane concentration: 0.75%), halothane administration was stopped and the rate of decrease in alveolar concentration of halothane (FE/FE0FE: measured end-tidal concentration of halothane; FE0: the endtidal concentration immediately preceding the cessation of halothane administration) was measured continuously. The rate of decrease in FE/F0was more rapid when nitrous oxide (70%) is discontinued abruptly and replaced by the same concentration of nitrogen (Part 2) than when the nitrous oxide is continued (Part 1). One minute and a half after the cessation of halothane administration, FE/FE0, was 0.38 ± 0.05 (mean ± SD) in Part 2 and 0.45 ± 0.04 in Part 1 (P< 0.01). In Part 2, the fall in the alveolar concentration of halothane was accompanied by a decrease in alveolar carbon dioxide from 4.27 ± 0.01% to 4.16 ± 0.01 % at 1.5 min and an increase in the mean expired tidal volume from 522 ± 39 ml to 557 ± 29 ml. The authors conclude that the elimination of nitrous oxide accelerates the elimination of halothane both by dilution and by an increased expired ventilation.