An arrangement is described of a plug with a sealing system for pressure testing of bore holes and the like in a formation or the like, comprising a pipe in which the plug is fitted in a plug-carrying chamber, and the plug closes the passage through the pipe in cooperation with sealing bodies, as the plug is arranged (rests) in a seat in the chamber. The arrangement is characterised in that the sealing bodies (23,25) are arranged in connection with the inner wall of the pipe (10) above (upstream) and/or below (downstream) of the chamber (30), and are arranged to form a seal against the respective cylindrical extensions (44,46) of the plug body (45) above and/or below the chamber.