An intravenous injection site for connection to a cannula, said injection site having proximal and distal ends, said injection site comprising: a support body presenting a well at a proximal end of the support body, a fluid passageway formed through the support body, and a pressure-actuated flow control valve spaced distally from the well and operable to selectively prevent fluid flow in a proximal direction towards the proximal end of the injection site, and a split septum unit for receipt in the well; characterized in that said pressure-actuated flow control valve has a dome-shaped, concavo-convex central body with a peripheral flange and a slit formed in the central body of the flow control valve, said central body being of progressively decreasing thickness leading to an apex of the central body; and that said split septum unit comprises a rigid septum holder and a resilient septum body mounted within the septum holder, wherein said well has an annular septum-engaging surface for restricting distal displacement of the septum body when the cannula is inserted into the septum body and for precompressing the septum body prior to cannula insertion, said septum holder including opposed, annular, distal and proximal ends (66a, 66b) disposed about said septum body and an annular inwardly projecting rib, said septum body presenting proximal and distal faces (74, 76), a slit between the proximal and distal faces (74, 76), and an annular groove, wherein the septum body is interfit with the septum holder, such that the annular groove of the septum body is mated with the annular inwardly projecting rib of the septum holder and wherein the proximal face of the septum body is coplanar with the proximal end of the septum holder prior to insertion of the cannula to present a substantially smooth swabable proximal face of the septum body, wherein upon insertion of the cannula, the septum body protrudes proximally without any significant distal displacement. a support body presenting a well at a proximal end of the support body, a fluid passageway formed through the support body, and a pressure-actuated flow control valve spaced distally from the well and operable to selectively prevent fluid flow in a proximal direction towards the proximal end of the injection site, and a split septum unit for receipt in the well; characterized in that said pressure-actuated flow control valve has a dome-shaped, concavo-convex central body with a peripheral flange and a slit formed in the central body of the flow control valve, said central body being of progressively decreasing thickness leading to an apex of the central body; and that said split septum unit comprises a rigid septum holder and a resilient septum body mounted within the septum holder, wherein said well has an annular septum-engaging surface for restricting distal displacement of the septum body when the cannula is inserted into the septum body and for precompressing the septum body prior to cannula insertion, said septum holder including opposed, annular, distal and proximal ends (66a, 66b) disposed about said septum body and an annular inwardly projecting rib, said septum body presenting proximal and distal faces (74, 76), a slit between the proximal and distal faces (74, 76), and an annular groove, wherein the septum body is interfit with the septum holder, such that the annular groove of the septum body is mated with the annular inwardly projecting rib of the septum holder and wherein the proximal face of the septum body is coplanar with the proximal end of the septum holder prior to insertion of the cannula to present a substantially smooth swabable proximal face of the septum body, wherein upon insertion of the cannula, the septum body protrudes proximally without any significant distal displacement. that said split septum unit comprises a rigid septum holder and a resilient septum body mounted within the septum holder, wherein said well has an annular septum-engaging surface for restricting distal displacement of the septum body when the cannula is inserted into the septum body and for precompressing the septum body prior to cannula insertion, said septum holder including opposed, annular, distal and proximal ends (66a, 66b) disposed about said septum body and an annular inwardly projecting rib, said septum body presenting proximal and distal faces (74, 76), a slit between the proximal and distal faces (74, 76), and an annular groove, wherein the septum body is interfit with the septum holder, such that the annular groove of the septum body is mated with the annular inwardly projecting rib of the septum holder and wherein the proximal face of the septum body is coplanar with the proximal end of the septum holder prior to insertion of the cannula to present a substantially smooth swabable proximal face of the septum body, wherein upon insertion of the cannula, the septum body protrudes proximally without any significant distal displacement.