Abstract: This invention relates to an improved arterial catheterization system which is designed to allow the user to quickly visualize the early flashback of blood after penetration of a vessel. The improved arterial catheterization system of this invention allows for minimum blood loss during initial penetration of the blood vessel. Specifically, this invention is directed to a system for introducing an over-the-needle catheter into a blood vessel, and comprises an inserter unit having a hollow needle and a catheter having a lumen such that the catheter is adapted to be carried on the needle. The catheter and needle cooperate to form a first flashback chamber therebetween. The first flashback chamber is at least partially translucent and allows for the visualization of blood flow upon insertion of the needle into a blood vessel. A guide wire, which is slidably mounted on the inserter unit within the needle, is dimensioned in relation to the needle in order to provide a blood flow passage therebetween.