A gate valve for use with a catheter including an elongated valve body having a transversely extending slide chamber and a slide member slidably supported in the slide chamber for transverse sliding movement between first and second positions. The slide chamber is defined in part by a pair of longitudinally spaced valve surfaces having longitudinally aligned inlet and outlet ports with the slide member having an enlarged opening extending therethrough also in a direction longitudinally of the valve body. A resilient insert element is carried within the opening in the slide member and is longitudinally moveable and self-adjustable therein with a longitudinal dimension in an uncompressed state greater than the distance between the valve surfaces of the slide chamber. The resilient insert element is supported in a state of longitudinal compression within the slide chamber and is in continuous sealing contact with the opposing valve surfaces regardless of whether the slide member is in its first position, or its second position, or is being moved between these positions. A longitudinal bore of the insert element is disposed in alignment with the inlet and outlet ports when the slide member is in its first position and is positioned out of alignment with these ports when the slide member is in its second position.