The present invention provides a needle-free injection device for delivering a medicament under pressure into an animal or human. The injection device includes an actuating device comprising: a gas tight chamber; a piston and rod assembly slidably received in the chamber and movable between a forward position and a rearward position; a gas charge in the chamber for urging the piston and rod assembly to the forward position; means for moving said piston and rod assembly against said gas charge into the rearward position; and a trigger for releasably retaining the piston and assembly in the rearward position. Also provided is a method and kit for using the device to administer a liquid through the skin of an animal or human.