Abstract: A catheter attachment to be utilized in conjunction with an arterial catheter for blood sampling purposes is disclosed. The attachment includes a housing adapted to be fixedly secured to the catheter, and a stopcock operatively connected to the housing for controlling the flow of blood to a sampling syringe. A sealing stylet is also disposed within the housing in a reciprocal manner so as to be axially inserted within, or withdrawn from, the catheter during non-sampling and sampling periods. In order to insure the disposition of the stylet within the catheter during non-sampling periods, a locking mechanism is also provided which prevents the axial displacement or withdrawal of the stylet from the catheter.