Abstract: New and improved single limb breathing circuit assembly for use in a circle absorbed anesthetic system is disposed within and surrounded by the expiratory limb to provide for heating of the inhaled gases by the exhaled gases. Valve means insure that substantially all exhalation occurs in the expiratory limb and that substantially all inhalation occurs in the inspiratory breathing limb to prevent re-breathing of exhaled carbon dioxide. The absorber comprises inlet and internal outlet conduits which combine to assure the flow of gases from the absorber at high humidity levels. Valve means are also provided to proportion the fresh gas inflow to the absorber between the inlet and outlet conduits to enable control of the humidity of the gases flowing from the absorber within a predetermined range. The various portions of the system are arranged so as to minimize both heat and moisture loss in the inhaled gases passing through the absorber for conveyance to the patient.