The invention relates to an annular die (1) on an extrusion head for producing tubular or sheet-like preforms with an arcuate die gap and with a melt channel (4) delimited by a mandrel (2) and a die body (3), the cross-sectional profile of the melt channel (4) being variable by way of adjusting elements, the annular die according to the invention being distinguished by the fact that the outer circumference of the melt channel (4) is at least partially delimited directly by at least one radially adjustable slide (5), that the slide (5) is formed by a number of segments that can be adjusted together from at least one first position into at least one second position, and that the segments in each position form a respectively closed arc portion of the delimitation of the melt channel (4), the radius of the arc portion being different according to the position of the segments.