An endoprosthesis for the knee joint arthrodesis with a. a femur part having a femur connection structure for a fixed connection with the distal end of the femur, b. a tibial part having a tibia connection structure for fixed connection with the proximal end of the tibia; c. a femur-side coupling portion formed on the femur part, d. a tibia-side coupling portion formed on the tibia part, wherein the femur-side coupling section and the tibia-side coupling section, are pivotally connected relative to one another about a prosthesis pivoting axis, which extends essentially horizontally from medial to lateral as knee replacements in the mounting position of the endoprosthesis and thus essentially corresponds to the position of the pivot axis of the natural knee, e. at least one locking element (15, 16) movable back and forth between a locking position in which it rigidly locks the femur-side coupling portion and the tibia-side coupling portion with one another in a predeterminable pivot position, and a release position, enabling pivoting of the femur-side coupling portion and the tibia-side coupling section relative to one another around the prosthesis pivot axis, f. a holding and resetting means, which forces the locking element (15, 16) with a holding force into the locking position and holds it therein, and g. a release mechanism by means of which the locking element (15, 16) is displaceable against the retaining force from the locking position into the release position, characterised in that one of the coupling portions, the femur-side coupling section or the tibia-side coupling section, has a hollow-cylinder portion which is closed on both ends and has a circular cross-section, which is oriented along the prosthesis pivoting axis with its cylinder longitudinal axis, and the other coupling portion, i.e. the tibia coupling portion or the femur-side coupling portion, includes a bearing portion, in the form of a fillet, extending parallel to the prosthesis pivoting axis, for receiving the hollow cylinder portion in a rotating manner around the cylinder longitudinal axis, and a stop wall extending perpendicular to the prosthesis pivoting axis and forming a front stop for the hollow cylinder portion, wherein at least two locating bores parallel to the prosthesis pivoting axis are provided in the stop wall at different positions, wherein the locking element (15, 16) include a piston plate provided mobile in an internal space of the hollow cylinder portion in the cylinder longitudinal direction, on which at least one locking spigot is provided eccentrically, extending in the cylinder longitudinal direction, protruding from the piston plate in the direction of the stop wall, wherein the locking spigot penetrates with its free end into an opening in the front side of the hollow cylinder portion facing the stop wall and in a relative pivoting position of both coupling portions (7, 8) in which it is flush with one of the locating bores in the stop wall, in which it can protrude and into it which it can retract by moving the piston plate in the cylinder longitudinal direction. a. a femur part having a femur connection structure for a fixed connection with the distal end of the femur, b. a tibial part having a tibia connection structure for fixed connection with the proximal end of the tibia; c. a femur-side coupling portion formed on the femur part, d. a tibia-side coupling portion formed on the tibia part, wherein the femur-side coupling section and the tibia-side coupling section, are pivotally connected relative to one another about a prosthesis pivoting axis, which extends essentially horizontally from medial to lateral as knee replacements in the mounting position of the endoprosthesis and thus essentially corresponds to the position of the pivot axis of the natural knee, e. at least one locking element (15, 16) movable back and forth between a locking position in which it rigidly locks the femur-side coupling portion and the tibia-side coupling portion with one another in a predeterminable pivot position, and a release position, enabling pivoting of the femur-side coupling portion and the tibia-side coupling section relative to one another around the prosthesis pivot axis, f. a holding and resetting means, which forces the locking element (15, 16) with a holding force into the locking position and holds it therein, and g. a release mechanism by means of which the locking element (15, 16) is displaceable against the retaining force from the locking position into the release position,