An elongate guidewire for use with a free, unsecured, intervascular device having an elongate, enclosed, open ended channel for receiving the guidewire. The guidewire includes an elongate, flexible body having an outer dimension which is less than the inner dimension of the channel and an expandable and contractable stop member mounted on the guidewire body for movement between a first contracted position and a second expanded position. The expandable and contractable stop member is dimensioned in the first contracted position to move through said channel and in the second expanded position to preclude movement of said stop member through the channel.