An I.V. flush syringe (20) assembly includes a barrel (22) having an insidesurface (32) defining a chamber (33) for retaining fluid, an open proximal end(28) and a distal end (30) including a distal wall (31) with an elongate tip(36) extending distally therefrom having a passageway (38) therethrough influid communication with the chamber (33). A plunger (24) having an elongatebody portion (25) and a stopper (54) slidably positioned in fluid-tightengagement with the inside surface (32) of the barrel (22) is provided. Anti-reflux structure for controlling stopper deflection when fluid has beendelivered from the chamber (33) and the stopper (54) is in contact with thedistal wall (31) is provided. The anti-reflux structure includes the stopper(54) having a conically shaped distal surface (59) and the inside surface (32)of the barrel (22) and the distal wall (31) being conically shaped (35)wherein the total included angle A of the inside surface (32) of the barrel(22) at the distal wall (31) is greater than the total included angle B of thedistal surface (59) of the stopper (54) by at least 6~.
