Abstract: The present invention comprises a device for advancing a guide wire into a blood vessel of a patient, which comprises a housing from a distal end of which extends a needle hub support having a blood aspiration canal extending along its length; a syringe chamber located in the housing; a passageway extending between the blood aspiration canal and the syringe chamber for the passage therethrough of aspirated blood; a switch mounted for sliding movement along a surface of the housing and having attached thereto a cannula for movement therewith; the switch being formed with a through hole aligned with the cannula and the blood aspiration canal for the passage therethrough of a guide wire; the arrangement being such that movement of the switch towards the distal end of the housing will cause the cannula to block the passage of blood along the passageway between the blood aspiration canal and the syringe chamber.