Needleless injector rod assembly comprising a two-piece tubular cylinder that includes a front section (5) and a rear section (6)； a first chamber (44) at a front discharge end of said front section (5)； a unidirectional valve (47) in said front section (5) to receive an injectable liquid from a source and feed the liquid into said first chamber (44)； a nozzle (12) at a front end of said cylinder for discharging liquid from the first chamber (44)； a plunger (11) that can slide in the front cylinder section (5) for movement between a retracted position in which liquid is drawn into the first chamber (44) and an extended position in which the liquid is discharged through said nozzle (12)； a second chamber (111) in said rear section (6) of the cylinder behind and separated from said first chamber (44)； a piston assembly (97) that can slide in said second chamber (111) for movement under fluid pressure between an extended position in which the piston (11) moves to the extended position and a retracted position in which the piston (11) to a retracted position； a third chamber (133) in said cylinder (6) surrounding said second chamber (111) to alternately receive pressurized gas from said second chamber (111) during the movement of the piston assembly (97) from the retracted position to the extended one, and to discharge the pressurized gas to said second chamber (111) during the movement of said piston assembly (97) from the extended position to the retracted one； a safety plug (95) that closes a rear end (86) of said rear section (6) of the cylinder and includes a first conduit (124) to receive a pressurized gas from a source (165) thereof, and that brings the gas to a rear end of the piston assembly (97) to cause the piston assembly (97) to move from the retracted to the extended position, and a second conduit (126) to bring the gas under pressure to and from said third chamber (133) to cause said piston assembly (97) to move alternately from the retracted to the exten
