Abstract: A rotary fluid-control valve for controlling fluid flow has a cylindrical valve chamber, a dove-tail shaped valve body mounted rotatably in the valve chamber and movable between an open position and a closed position, and four fluid passages extending from the valve chamber. The passages are arranged essentially parallel with respect to one another and the valve body is pivotally connected to the valve chamber with a pivot pin extending in the axial direction of the valve chamber. The passages extend perpendicular to the pivot pin. The valve chamber has a convex section that, when the valve body is rotated out of the closed position, causes an asymptotic rise in the throughflow quantity. A stop or pressure equalizing projection projects into the valve chamber and is dimensioned so that a pressure drop in the closed position of the valve body corresponds to a pressure drop of the radiator in the open position of the valve body.