Abstract: A plug or stopper for bores in rocks or the like, which bores, after the lling, are filled with a self-hardening synthetic material, such as commercially available epoxy resin mixtures, polyester resin mixtures, and cyanoacrylate adhesives. The plug is pressed or driven into the bore and consists essentially of an elastically deformable material, especially rubber or rubber-like synthetic resin. The outer diameter of the plug corresponds to the diameter of the rock bore. The plug tapers mainly toward the front and is provided on its outer circumference with grabbing elements. The inside of the plug is provided with an axial bore which becomes narrower toward the back. A closure member which serves as a valve body is axially displaceably arranged in the plug in such a way that it seals off the axial bore when pressure is directed toward the back.