Abstract: A motion compensation system is provided for use on a drilling vessel that is subject to heaving due to wave action, and wherein a motion compensator associated with a derrick supports a drill string that extends from the vessel downwardly through the seabed and into a drilled hole in the sub-sea formation, the system including a first encoder for sensing vertical motion of the vessel, due to sea heaving, and for producing an electrical signal that varies in response to such sensing, a device on the vessel for producing hydraulic fluid displacement corresponding to variations of the signal, and an hydraulic fluid responsive manipulator in the form of an actuator or actuators carried by the derrick for lifting and lowering the compensator in correspondence to such hydraulic fluid displacement, thereby to maintain the drill string substantially motionless, vertically.