Mandrel rod (1) for pipe rolling mills, in which hollow billet (5) is rolled on mandrel rod (1), consists of two pipes (2, 3) lying inside one another. Annular gap (4) between pipes (2, 3) is filled with a thermosetting plastic, whose coefficient of expansion is selected large enough so that the thermosetting plastic completely fills annular gap (4), even with temperature differences, between pipes (2, 3).