Abstract: There is disclosed herein a catheter introducer having an axially disposed cylindrical elastic segment which covers first and second substantially solid cylindrical elements each defining internal and external diameters, each of said annular elements, as well as the elastic segment, disposed radially symmetrically about a common axis. Also provided are means for rotatably securing said first and second elements relative to said common longitudinal axis for facilitating a reduction in diameter of said elastic segment in order to perform a fluid-tight seal against any elongated element placed along and within said longitudinal axis. A catheter so introduced may be advanced, withdrawn, held, or otherwise manipulated without loss of the fluid or atmospheric seal between the internal diameter of the elastic segment and the object introduced.