Abstract: A portable ventilating device for ventilating patients in the field. A reciprocating piston divides a cylinder into a pressure chamber and a spring chamber in which a compression spring acts against the piston. Gas pressure applied to the pressure chamber opposes the spring force and reciprocates the piston. As the piston reciprocates ventilating gas is forced out of the piston chamber through a ventilation line equipped with a check valve. Alternative embodiments provide gas pressure assistance to the spring, make use of the gas which powers the device as the ventilating gas, and provide adjustment of the duration for the time periods gas is applied to the cylinder.