Abstract: Medical ventilating apparatus is provided whereby gas is supplied to a patient at a preset volume flow rate, with the patient breathing spontaneously what he can from this flow, while the remaining gas is accumulated and forcibly delivered intermittently at preset volumes. The apparatus comprises a primary reservoir to hold gas at a constant pressure for spontaneous breathing, and a secondary reservoir in which accumulated gas is held for forced ventilation. Two basic forms of apparatus are possible in each of which one reservoir is filled up to a preset volume and forced ventilation is triggered when the other reservoir has also filled to a preset volume. In one form the reservoirs are connected in parallel and triggering is effected by the secondary reservoir, and in the other form the reservoirs are connected serially and triggering is effected by the primary reservoir.