A preform (1) of plastic material for manufacturing bottles through a blow-moulding process includes a substantially cylindrical central body (3) having an open end (2) and a closed end (4). In correspondence of the central area of the closed end (4) there is provided a hollow appendix (5), projecting outwards of the preform, capable of internally housing the tip end of a stretching rod of the preform (1). Both the hollow appendix (5) and the remaining side portion of the closed end (4) of the preform may be spherical, conical, elliptical or parabolic.