Abstract: A heave-compensating device particularly for floating oil drilling rigs incorporates a passive load-supporting system capable of being set to exert a chosen datum load-supporting force and an active force-modifying system operative when the force exerted by the passive load supporting system deviates from the chosen datum force to reduce the deviation. The passive load-supporting system may include a cylinder and piston assembly connected between a fixed support to which said system is connected and the load. The end of the cylinder adjacent the load is connected to a supply of liquid and to one end of a dashpot cylinder containing a piston, the other end of the dashpot cylinder being connected to a supply of compressed gas.