A catheter assembly (2, 102, 202) comprising: a catheter hub having a wall having an inner surface and a chamber therein, the catheter hub having a distal end and a proximal end, the catheter hub being connectable at the proximal end of the catheter hub to a device for the infusion or withdrawal of fluids to or from the chamber within the catheter hub; a hollow tubular catheter having an interior, a proximal end and a distal end, the catheter being connected at the proximal end of the catheter to the distal end of the catheter hub, the interior of the catheter opening into the chamber within the catheter hub; a needle having a shaft, a distal end portion and a sharpened needle tip, in a ready position the needle extending through the chamber in the catheter hub and the catheter with the needle tip extending beyond the distal end of the catheter; a valve assembly disposed within the chamber of the catheter hub, the valve assembly comprising: a first valve member; and a second valve member preventing the flow of fluid through the chamber to or from the proximal end of the catheter hub, the second valve member being a two-way valve and openable to permit the flow of fluid through the chamber in the catheter hub in both a proximal and a distal direction; characterised by further comprising: a hollow flexible extension tube having a distal end, an opening in the distal end and a proximal end, the extension tube being connected at the distal end of the extension tube to the catheter hub between the proximal and distal ends (6, 8) thereof, the flexible extension tube opening at the distal end of the extension tube into the chamber within the catheter hub, the extension tube connectable at its proximal end to a device for infusing fluid into the chamber within the catheter hub; wherein the proximal end of the extension tube is provided with a connection hub; and wherein the first valve member closes the distal end of the flexible extension tube and is openable under an action of a pressurised fluid within the flexible extension tube. a catheter hub having a wall having an inner surface and a chamber therein, the catheter hub having a distal end and a proximal end, the catheter hub being connectable at the proximal end of the catheter hub to a device for the infusion or withdrawal of fluids to or from the chamber within the catheter hub; a hollow tubular catheter having an interior, a proximal end and a distal end, the catheter being connected at the proximal end of the catheter to the distal end of the catheter hub, the interior of the catheter opening into the chamber within the catheter hub; a needle having a shaft, a distal end portion and a sharpened needle tip, in a ready position the needle extending through the chamber in the catheter hub and the catheter with the needle tip extending beyond the distal end of the catheter; a valve assembly disposed within the chamber of the catheter hub, the valve assembly comprising: a first valve member; and a second valve member preventing the flow of fluid through the chamber to or from the proximal end of the catheter hub, the second valve member being a two-way valve and openable to permit the flow of fluid through the chamber in the catheter hub in both a proximal and a distal direction; a first valve member; and a second valve member preventing the flow of fluid through the chamber to or from the proximal end of the catheter hub, the second valve member being a two-way valve and openable to permit the flow of fluid through the chamber in the catheter hub in both a proximal and a distal direction; a hollow flexible extension tube having a distal end, an opening in the distal end and a proximal end, the extension tube being connected at the distal end of the extension tube to the catheter hub between the proximal and distal ends (6, 8) thereof, the flexible extension tube opening at the distal end of the extension tube into the chamber within the catheter hub, the extension tube connectable at its proximal end to a device for infusing fluid into the chamber within the catheter hub; wherein the proximal end of the extension tube is provided with a connection hub; and wherein the first valve member closes the distal end of the flexible extension tube and is openable under an action of a pressurised fluid within the flexible extension tube.