Pre-rotational swirl controller for rotary pumps having a flow duct enclosed by a casing and a plurality of guide vanes located substantially radially within it. Each of these has two bearings, i.e. one on a hub located centrally and one on a trunnion which passes through the casing and which is also acted upon by an adjusting device. The guide vanes can be radially assembled through the casing because, for each guide vane, there is a removal slot which is closed by a cap which carries the seal and the bearing for the guide-vane trunnion, these being designed so as to be tolerant of alignment errors. A self-locking setting gear with a large transmission ratio and a through drive shaft is provided for the adjustment.