A synchronized intermittent mandatory ventilation respirator for providing inhalation and exhalation cycles to a patient while permitting the patient to carry out a complete inhalation and exhalation cycle spontaneously, comprising a gas supply adapted to be connected to a patient for supplying breathing gas under pressure to the patient, a main line connected to the gas supplied for supplying breathing gas to the patient, a main valve in the main line, and a timing system connected to the gas supply and the main valve for opening the main valve for a first selected timed inhalation cycle to supply breathing air to the patient and closing the valve for a selected timed exhalation cycle to permit the patient to exhale. A timer is connected to the main line downstream of the main valve, to the gas supply and to the timing system for blocking the connection between the timing system and the gas supply for stopping cycling of the timing system in response to a spontaneous inhalation of the patient and for supplying breathing air to the patient for a second selected timed inhalation cycle, and a trigger is connected to the main line downstream of the main valve and to the timing system for switching the timing system from an exhalation cycle to an inhalation cycle in response to a spontaneous inhalation of the patient.