A self-locking syringe assembly is disclosed. The syringe assembly includes a barrel having an interior surface. An elongate plunger shaft is axially moveable within the barrel. The plunger shaft includes at least two longitudinally extending splines. A stopper is attached to the distal tip of the plunger shaft which provides a fluid-tight seal with the barrel. Tabs are positioned at annularly spaced locations on the interior surface of the barrel. The tabs extend radially inwardly from the sidewall a predetermined distance and define recesses therebetween for receiving the plunger splines during axial movement of the plunger shaft within the barrel. The splines include recesses to enable rotational movement of the plunger shaft when the recesses are axially aligned with the tabs to dispose the splines against the tabs. In this manner, the plunger shaft may be locked in a predetermined longitudinal position relative to the barrel. Once in this locked position, the plunger shaft is prevented from being pulled or pushed towards the distal end of the barrel.
