Abstract: An autodiscardable hypodermic syringe having a preset piston stroke limitation includes an elongated cylinder having an opened end an opposite substantially closed end. A conduit of an external tubular mouth piece for coupling with the needle is connected to the substantially closed end of the cylinder. A piston is slidably received within the cylinder. A rod for sliding the piston is selectively connected to the piston. An end of the rod remote from the piston projects beyond the open end of the cylinder. The piston is selectively connected to the rod through a coupling and uncoupling mechanism. The coupling and uncoupling mechanism includes a narrow body that projects from the internal end of the rod and has a passage coincident with a central axis of the rod. The passage extends from side to side of the narrow body and defines a descending section that in its middle part has a deviation disposed outward of the descending section and of the narrow body.