Abstract: A vaporizer has a main conduit supplied at an inlet end by a carrier gas of oxygen and nitrous oxide having a constant flow rate. The outlet end of the conduit is connected to the breathing system of a patient. A series circuit with an input end connected to a proximal port in the conduit includes a check valve, a chamber in which anesthetic vapor is maintained in saturation and a positive displacement pump driven by an electrical motor for pumping the vapor-saturated gas at a rate that is only a function of an electrical signal applied to the motor. The output of the pump is connected to a distal port of the conduit. More than one such chamber may be provided, each containing a different liquid anesthetic, and any one being connected into the circuit by means of a selector valve.