A syringe assembly comprising: a barrel including a cylindrical side wall having an inside surface defining a chamber for retaining fluid, an open proximal end and a distal end including a distal wall having a passageway therethrough in fluid communication with said chamber; an elongate hollow plunger rod having a proximal end, an open distal end, an interior surface, and a distal lip on the distal end of said plunger rod; a stopper at the distal end of the plunger rod, the stopper including a sealing element having a peripheral surface forming a seal with said inside surface of the barrel, and a projection extending proximally from the sealing element and forming a longitudinal recess including a distal portion, the longitudinal recess including a plurality of ramps (118, 120) disposed on the distal portion of the longitudinal recess and inclined outwardly in a distal direction and a plurality of proximal recess detents (140, 142, 144); a locking element including a proximal portion having a proximal edge and a distal portion having an outwardly facing barb, and the locking element is positioned in the longitudinal recess so that the barb contacts the interior surface of the plunger rod and the proximal edge contacts the proximal recess detent (140, 142, 144), wherein proximal sliding movement followed by distal sliding movement of the plunger rod and stopper within the chamber causes the barb to extend past the distal lip on the plunger rod and toward the inside surface of the barrel, preventing further proximal movement of the plunger rod with respect to the chamber, characterized in that the plurality of ramps (118, 120) and the plurality of proximal recess detents (140, 142, 144) on the longitudinal recess cooperate so that the distal portion of the locking element rides up the plurality of ramps (118, 120) and the barbs of the locking element to move the distal portion of the locking element outwardly towards the interior surface of the plunger rod during distal sliding movement of the plunger rod and stopper. a barrel including a cylindrical side wall having an inside surface defining a chamber for retaining fluid, an open proximal end and a distal end including a distal wall having a passageway therethrough in fluid communication with said chamber; an elongate hollow plunger rod having a proximal end, an open distal end, an interior surface, and a distal lip on the distal end of said plunger rod; a stopper at the distal end of the plunger rod, the stopper including a sealing element having a peripheral surface forming a seal with said inside surface of the barrel, and a projection extending proximally from the sealing element and forming a longitudinal recess including a distal portion, the longitudinal recess including a plurality of ramps (118, 120) disposed on the distal portion of the longitudinal recess and inclined outwardly in a distal direction and a plurality of proximal recess detents (140, 142, 144); a locking element including a proximal portion having a proximal edge and a distal portion having an outwardly facing barb, and the locking element is positioned in the longitudinal recess so that the barb contacts the interior surface of the plunger rod and the proximal edge contacts the proximal recess detent (140, 142, 144), wherein proximal sliding movement followed by distal sliding movement of the plunger rod and stopper within the chamber causes the barb to extend past the distal lip on the plunger rod and toward the inside surface of the barrel, preventing further proximal movement of the plunger rod with respect to the chamber, characterized in that the plurality of ramps (118, 120) and the plurality of proximal recess detents (140, 142, 144) on the longitudinal recess cooperate so that the distal portion of the locking element rides up the plurality of ramps (118, 120) and the barbs of the locking element to move the distal portion of the locking element outwardly towards the interior surface of the plunger rod during distal sliding movement of the plunger rod and stopper.