The present invention provides a pressure-operated hypodermic injection apparatus which is particularly suitable for needleless operation. The apparatus includes an elongate shell having an inwardly facing wall with a uniform configuration along a defined length thereof, with the shell further defining a discharge aperture adjacent one end thereof. It further includes a plunger which is slideably disposed within the defined length of the shell, and which is shiftable between a ready position and a spent position, with the spent position being closer to the aperture than the ready position. With the plunger in its ready position, the shell and plunger collectively define an ampule chamber which is capable of containing a dischargeable medication. A pressurized-gas cartridge is mounted to the plunger to shift therewith, and has gas-releasing means which faces away from the discharge aperture. A body is further included which is sealingly and releasably joined to the other end of the shell. An actuator is mounted to the body and is manually operable for selectively operating the gas-releasing means to drive the plunger from its ready toward its spent position to discharge medication from the apparatus and into the patient.