A catheter end closure assembly includes a catheter adaptor with a head, a stem, a fastening segment between the head and stem and an axial passage extending the length of the adaptor. A plug with a head and a shank is shaped and dimensioned so that the shank may slidably engage in the stem and block a segment of the passage therein. The assembly also includes a cap dimensioned to be engaged around the stem and the fastening segment so as to define between the cap and adaptor a closed chamber that contains the stem and plug. Interfitting fastening surfaces on the cap and the fastening segment may releasably couple the cap to the adaptor, and cooperating surfaces on the plug and cap side wall may interfit when the cap is coupled to the adaptor so that when the cap is subsequently decoupled from the adaptor, the plug is automatically pulled from the passage. The assembly may be incorporated with a catheter extension set to facilitate dialysis or other therapy.