Abstract: An implantable device for use in a human and/or animal body for replacement of an organ valve. The device has an elongate main body having a first end and a second end provided with openings, and a membrane element provided with at least one opening. The device has, in a first operating state (primary form), a large ratio of length to transverse extent along an axis, and, in a second operating state (secondary form), a smaller ratio of length to transverse extent along an axis. The device is reversibly convertible from the secondary form into the primary form by application of force against elastic material forces. At least one of the two ends of the main body has an outwardly protruding anchoring portion for anchoring the device in the organ and/or a vessel.