1. A volume-cycled respirator system for delivering a controlled volume of gas to a patient, the system comprising: a. displaceable gas delivery means having a position which is adjustable for forcing periodically a volume of gas under pressure into the lungs of a patient throughout an inspiratory period of a respiratory cycle; b. drive means operatively connected to the gas delivery means to displace the volume of gas to be delivered; c. means for generating a reference signal representing variations in the position of the gas delivery means with respect to time throughout the inspiratory period necessary to deliver a desired volume of gas in accordance with a desired volume-versus-time waveform; and d. closed loop feedback means for controlling the instantaneous position of the gas delivery means throughout the inspiratory period, the closed loop feedback means including 1. first means responsive to the actual position of the gas delivery means for generating a position feedback signal representing the actual volume of gas displaced by the gas delivery means with respect to time throughout the inspiratory period, 2. second means responsive to said reference signal and said position feedback signal for providing throughout the inspiratory period a position error signal representing the deviation between the instantaneous desired position and the corresponding actual position of the gas delivery means, and 3. third means for applying said position error signal to the drive means for adjusting the instantaneous position of the gas delivery means in proportion to said error signal throughout the inspiratory period to deliver gas to the patient in accordance with the desired volume-versus-time waveform.