Abstract: An assembly is provided for easing the insertion and placement of an arterial catheter in the blood vessel of a patient. The assembly includes the combination of an insertion needle and guide tube which pass through the lumen of a catheter, and prior to use extends slightly out of the distal end of the catheter. The needle, in turn, extends slightly out of the distal end of the guide tube and serves to make the initial insertion through the skin and into the blood vessel. Once initial insertion is made, as indicated by blood "flash back" to the transparent needle hub, the guide tube is advanced through the catheter to provide a tracking path for the catheter. Once the distal end of the catheter has followed the guide tube path, the guide tube and needle are withdrawn. A feature of the invention is a guide tube slide arrangement for controlling the advance and retraction of the guide tube.