Ventilator apparatus comprises a cyclically-operated reciprocating pump, a delivery tube for delivering pressurized air to a patient, an exhalation valve assembly producing breathing cycles in which the delivery tube is first connected to the patient to effect inhalation and then is vented to the atmosphere to permit exhalation, and a control valve operated by the pump for controlling the breathing cycles of the exhalation valve. The pump has a low volume-displacement and operates at a relatively high cyclical speed, thereby enabling the apparatus to be selectively operated to perform the functions of any one of a large number of different types of ventilators.