A unidirectional injector having at least one cylinder-piston unit, the piston thereof being displaceable between two stroke end positions relative to the cylinder, wherein the cylinder-piston unit comprises at least one sealing system sealing a compensating space from the surroundings. To this end the piston is provided at its front surface with an axial annular groove for accommodating a seal ring or a permanently elastic seal material, that the cylinder is provided with a first seal element and a second seal element which are spaced by at least a piston stroke, or that the piston is provided with the first seal element and the second seal element which are spaced by at least a piston stroke, or the cylinder carries the first seal element and the piston carries the second seal element which is displaced with respect to the first by the piston stroke.
