Abstract: A syringe has a guide member internally associated therewith which can be advanced and retracted by actuation of the thumb of the hand which holds the syringe. A catheter is mounted on the needle of the syringe so that once the needle has been inserted into a body vessel and the lumen of the vessel has been located by manipulation of the guide member, the catheter can be moved off the needle and along the guide member into the lumen of the vessel. The apparatus is constructed and operated so that the guide member is maintained within the syringe prior to its use as a probe within the vessel. The apparatus is also constructed and operated so that the hands of the user of the apparatus need not be moved from their initial positions during the entering of the vessel and the probing for the lumen of the vessel. Additionally, the apparatus is constructed so that a suction can be applied to the fluid within the vessel to withdraw the fluid from the vessel to note when the vessel has been entered.