A split septum assembly for coupling with a miniaturized intravenous injection site used to administer fluids to a patient, the split septum assembly presenting an axial length extending along an axis of a fluid passageway of the intravenous injection site when the split septum assembly is coupled with the site, a transverse width, a distal end oriented towards a patient when the split septum assembly is coupled with the intravenous injection site, and a proximal end oriented towards a clinician administering fluids to the patient, the split septum assembly including a resilient and compressible split septum having an axially-formed slit for receipt of a blunt cannula, needle, or other medical device through said slit, a septum holder for receipt of the split septum, said septum holder having a generally cylindrically shaped body presenting a radial-most outer surface having an outer diameter and a radial-most inner surface having an inner diameter, and a septum housing for receipt of the combined septum holder and split septum, the septum housing having a proximal end that is open for positioning of the combined septum holder and split septum therethrough, and a distal end that is open to the fluid passageway of the injection site, said septum including a generally cylindrical body presenting a radial-most outer surface that has an outer diameter, and an annular flange circumscribing the radial-most outer surface of the septum body at a distal end of the septum body, wherein an axial length of the septum body is greater than an axial length of the flange, said flange extending radially from the septum body to present a radial-most outer edge of the flange having an outer diameter of the flange, wherein said outer diameter of the flange is greater than the outer diameter of the septum body, said flange including a proximally-extending locating projection annularly formed on the flange; said septum holder including an annular locking ring formed on the radial-most outer surface of the septum holder body and circumscribing the body, wherein said split septum body is configured to be positioned within the septum holder, such that the split septum flange extends beyond a transverse periphery of the septum holder; and said septum housing having a generally cylindrically shaped body having a radial-most outer surface and a radial-most inner surface presenting an inner diameter, a proximally-extending annular locating groove for receipt of the locating projection, and a radially-extending annular locking groove for interconnecting with the locking ring formed on the septum holder, wherein the locking ring presents a radial-most outer surface that has an outer diameter that is larger than the outer diameter of the radial-most outer surface of the septum holder body, wherein a shape of the locking ring in a pre-use position of the intravenous injection site is symmetrical in cross section, and a shape of the locking ring in an in-use position of the intravenous injection site is asymmetrical in cross section. said septum including a generally cylindrical body presenting a radial-most outer surface that has an outer diameter, and an annular flange circumscribing the radial-most outer surface of the septum body at a distal end of the septum body, wherein an axial length of the septum body is greater than an axial length of the flange, said flange extending radially from the septum body to present a radial-most outer edge of the flange having an outer diameter of the flange, wherein said outer diameter of the flange is greater than the outer diameter of the septum body, said flange including a proximally-extending locating projection annularly formed on the flange; said septum holder including an annular locking ring formed on the radial-most outer surface of the septum holder body and circumscribing the body, wherein said split septum body is configured to be positioned within the septum holder, such that the split septum flange extends beyond a transverse periphery of the septum holder; and said septum housing having a generally cylindrically shaped body having a radial-most outer surface and a radial-most inner surface presenting an inner diameter, a proximally-extending annular locating groove for receipt of the locating projection, and a radially-extending annular locking groove for interconnecting with the locking ring formed on the septum holder, wherein the locking ring presents a radial-most outer surface that has an outer diameter that is larger than the outer diameter of the radial-most outer surface of the septum holder body, wherein a shape of the locking ring in a pre-use position of the intravenous injection site is symmetrical in cross section, and a shape of the locking ring in an in-use position of the intravenous injection site is asymmetrical in cross section.