Abstract: An assembly for introducing a catheter into a blood vessel is provided. The assembly includes a needle hub having a needle secured thereto. The needle includes an elongate shaft having a bevelled tip. A portion of the needle shaft adjacent to the tip has a relatively large outside diameter. The shaft diameter of this portion exceeds the diameter, of the shaft portion adjoining the tip and the diameter of the shaft portion extending between the enlarged shaft portion and the needle hub. A catheter is mounted over the shaft of the needle, and includes an inner surface which bears against the enlarged shaft portion. A substantially leak-proof seal is thereby provided between the catheter and the needle shaft. A needle tip cover is slidably mounted to the needle shaft and is engageable with the enlarged portion of the shaft to prevent its removal therefrom. The catheter is releasably mounted to the needle tip cover.