Abstract: Apparatus for introducing a catheter into a patient's vein comprises a tubular housing having an axially extending guide slot extending from a point adjacent the forward end of the housing to a point adjacent the rear end of the housing. A tubular transparent mount for a hollow hypodermic needle is slidably mounted in the tubular housing with the sharpened end of the needle projecting forwardly. The overall length of the needle mount and needle is less than the length of the housing. A locking element rotatably mounted on the needle mount projects through the elongated slot and is selectively engaged with recesses provided in the forward and rear end wall portions of the elongated slot, by rotation relative to the needle mount, to lock the needle mount in an operative position with the needle projecting out of the housing, or an inoperative position with the needle retracted into the housing.