Abstract: An implantable blood pump comprises a flow passage (13) and two valves (15, 16), located spaced apart in the longitudinal direction of said passage (13) and permitting a flow of blood therethrough only in one direction and defining a pump chamber (14) therebetween, having an elastically deformable peripheral wall (17). The valve located at the inlet end of the pump chamber (14) being supported by a holder (19), arranged for reciprocating movement in the longitudinal direction of the pump chamber (14) by means of periodically operating driving means (24, 25). In order to reduce the risk of thrombosis within the pump chamber (14), said wall is formed by an elastically extensible hose (17), surrounded by a bellows (27), defining a liquid-filled annular space (29) around said hose (17) and being sealingly connected thereto at opposite ends thereof.