The invention discloses an artificial valve (10, 20, 30, 50) for implantation in a mammal body, in or adjacent to a mammal blood vessel, comprising a casing (12, 37, 61) and a closing mechanism, with at least part of said closing mechanism being a first moving part (11; 21, 22; 31, 32, 33; 51) adapted to make movements relative to said casing (12, 37, 61), said movements being to assume an open and a closed position for opening and closing, respectively, the blood flow through said blood vessel, as well as to positions in between said open and closed positions. In the valve of the invention, the closing mechanism is adapted to be powered in its movements to the closed position, with the powering being at least in part by means of a power mechanism (76, 83) external to the blood vessel.