The method makes it possible to produce a seal (1) of an extruded plastics material, including a tubular section (3) with a wall of variable thickness (WT), using a distribution chamber having a terminal die plate (12) with an annular extrusion orifice (15) formed therein, corresponding to the cross section of the tubular section (3). A chamber (18) is formed in the distribution chamber in communication with the extrusion orifice (13) and into which an elastomeric material is fed for forming the tubular section (3). The chamber (18) of the distribution chamber is in open communication with the outside environment by a passage (21) which is separate from the extrusion orifice (13) and is provided to carry a flow of elastomeric material coming from the chamber (18). During extrusion, the output cross section of the elastomeric material through the passage (21) is varied so as to cause corresponding variations in the pressure of the elastomeric material in the chamber (18) and corresponding variations in the thickness of the wall (WT) of the tubular section (3) extrud through the extrusion orifice (13).