Abstract: This invention relates to a non-return valve or check valve in the form of a piston-type slide-valve comprising a substantially rotationally-symmetrical outer housing and a likewise substantially rotationally-symmetrical inner housing which is arranged centrally therein and which is of streamline configuration, a free annular space serving as a flow passage between the outer and the inner housings which annular space is subdivided by connecting webs and can be shut off by a piston which can be extended coaxially from the inner housing, under spring pressure, against the direction of flow of a flow medium.