The reduction in the amount of methoxyflurane needed for anesthesia by addition of nitrous oxide was determined in 16 patients undergoing elective operations. Sixty per cent inspired nitrous oxide decreased methoxyflurane MAC to 0.070 per cent. Compared with the previously reported MAC value of 0.16 per cent for methoxyflurane alone, this represents a 56 per cent reduction in methoxyflurane anesthetic requirement. All patients moved when alveolar concentration was below 0.066 per cent; with one exception, no patient moved when alveolar methoxyflurane was above 0.072 per cent. It is estimated that the minimum alveolar concentrations of fluroxene (Fluoromar), halothane (Fluothane) and methoxyflurane (Penthrane) will be decreased about 1 per cent for every 1 per cent alveolar nitrous oxide.