The Humphrey circuit system function with the efficiency of the Lack system for spontaneous respiration and of the Bain system for controlled ventilation. It is designed to incorporate into a single system the advantages of the Mapleson A, D, and E type systems and to be available in coaxial and non-coaxial forms. This study was done to study the functions of a new anesthetic breathing system and to evaluate the adequacy of the recommended fresh gas flow. For spontaneous and controlled ventilation the system required a low fresh gas flow to achieve normocarbia(In the Lack circuit system, 60ml/min., is useded and in the Bain circuit system, 70ml/min.). The advantage of this system was that it was simple to operate and maintain, more easily sterilized and applicable in academic use. Further more the non-coaxial form of the circuit overcame the hazard of the inner tube complication. But this form was thought to have the problem of humidifying the inspired gas which still has to be sutdied. This multicircuit system was used satisfactorily with low fresh gas flow under spontaneous and controlled ventilation maintaining normal range of arterial carbon dioxide. It may become one of the preferred semiclosed breathing systems because of simplicity, multipurpose usefulness and other advantages.