An apparatus for extruding a polymeric material, having an extrusion head which includes a male die, a female die coaxially arranged with respect to the male die, a conveying channel, and at least one portion of which is defined between the male die and the female die. The apparatus further includes a device for adjusting a cross-sectional area of the at least one portion of the conveying channel by reciprocally displacing the female die with respect to the male die in response to an extrusion speed variation of the polymeric material.