Abstract: A blow-molded thermoplastic body is made by a method which comprises the following steps: inflating a hollow parison within a mold cavity for causing the parison to expand and conform to inner wall surfaces defining the mold cavity, whereby the blow-molded body is shaped; further inflating the blow-molded body within the mold cavity for causing a wall portion of the body to expand beyond the mold cavity into a port cavity communicating with the mold cavity thereby reducing the thickness of the wall portion; applying a counter pressure on an outer face of the wall portion for moving the wall portion back in the direction of the mold cavity against an inflating pressure prevailing within the blow-molded body, whereby connecting portions attaching the wall portion to the blow-molded body are reduced in thickness; and discontinuing the application of the counter pressure for allowing the wall portion to be driven anew away from the mold cavity into the port cavity by the inflating pressure.