Abstract: Apparatus for injecting or aspirating a fluid, including a liquid or paste. The apparatus includes a first piston adapted to be in direct contact with the fluid; and a second piston comprising a shaft adapted to coact with the first piston. The second piston is slideably mounted in a cylinder. The cylinder is divided by the second piston into a first chamber situated on the fluid injection side of the cylinder and a second chamber situated on the opposite side of the cylinder. The cylinder is adapted to be in fluid communication with a positive or negative pressure. At least one communication channel is positioned between the second chamber and the exterior of the cylinder. The at least one channel has a chamber orifice opening into the second chamber and an exterior orifice opening to the exterior. Blocking means for blocking the channel are also provided.