A single use hypodermic syringe, comprising: a barrel having a barrel wall defining a bore; a plunger slidably moveable within the barrel; a needle connector at the distal end of the barrel having a first portion within the barrel bore and a second portion extending from an opening in the distal end of the barrel, the needle connector being in sealing, releasable engagement with the barrel wall; a needle hub affixed to the needle connector and having a hypodermic needle fixed thereto; the needle connector and the needle hub forming an assembly, the assembly having a cavity therein, the needle and the barrel bore being in fluid communication through the cavity; an elastically-deformable sealing ring in the cavity having an opening therein; a sealing member in the cavity having a head and a shaft, the sealing member being moveable in the cavity, by means of fluid movement in the cavity, from a first position in which the head is on the proximal side of the ring to a second position in which the head is on the distal side of the ring; the ring being configured to stop movement of the sealing member from the second position to the first position and to form a seal with the sealing member, when the sealing member is in the second position, against a flow of fluid in the proximal direction; locking means for locking together the plunger and the needle connector; characterized in that the needle being retractable into the barrel bore, when the plunger and the needle connector are locked together, by movement of the plunger in the proximal direction[[.]]; and that the locking means comprises a plurality of latching members on the plunger at its distal face, spaced from the radially outer periphery thereof, and mating attachment means (316, 318, 330) for latching members in the needle connector; wherein the latching members have radially outwardly-extending projections; wherein the projections are subtended by a sloping surface on the outward surface of the latching members for engagement with an annular member of the needle connector; and wherein the mating attachment means comprises an axially-extending annular groove in the needle connector to receive the latching members and a radially-extending slot in the needle connector to receive the projections on the latching member. a barrel having a barrel wall defining a bore; a plunger slidably moveable within the barrel; a needle connector at the distal end of the barrel having a first portion within the barrel bore and a second portion extending from an opening in the distal end of the barrel, the needle connector being in sealing, releasable engagement with the barrel wall; a needle hub affixed to the needle connector and having a hypodermic needle fixed thereto; the needle connector and the needle hub forming an assembly, the assembly having a cavity therein, the needle and the barrel bore being in fluid communication through the cavity; an elastically-deformable sealing ring in the cavity having an opening therein; a sealing member in the cavity having a head and a shaft, the sealing member being moveable in the cavity, by means of fluid movement in the cavity, from a first position in which the head is on the proximal side of the ring to a second position in which the head is on the distal side of the ring; the ring being configured to stop movement of the sealing member from the second position to the first position and to form a seal with the sealing member, when the sealing member is in the second position, against a flow of fluid in the proximal direction; locking means for locking together the plunger and the needle connector; characterized in that the needle being retractable into the barrel bore, when the plunger and the needle connector are locked together, by movement of the plunger in the proximal direction[[.]]; and that the locking means comprises a plurality of latching members on the plunger at its distal face, spaced from the radially outer periphery thereof, and mating attachment means (316, 318, 330) for latching members in the needle connector; wherein the latching members have radially outwardly-extending projections; wherein the projections are subtended by a sloping surface on the outward surface of the latching members for engagement with an annular member of the needle connector; and wherein the mating attachment means comprises an axially-extending annular groove in the needle connector to receive the latching members and a radially-extending slot in the needle connector to receive the projections on the latching member.