Abstract: A device and method for accessing a vascular system of small laboratory animals. A port unit is configured to connect to an indwelling catheter of the animal. The port unit is secured to the animal by a harness block. At least one elastomeric band secures the harness block around the animal. A mating unit is configured with a cannula that pierces a septum at the top of the port unit. The catheter is connected to a conduit via a spring tether and allows a test substance to pass to the animal's vascular system from the conduit via a pathway through the cannula to the port unit and to the indwelling catheter. The septum prevents both forward and backward leakage and contamination of the indwelling catheter even after repeated connections and disconnections.