Abstract: A hydraulic drive system for actuating a reciprocating member such as a polished rod in a pump jack device, and for acting as a counterbalance and/or for energy conservation. The system has two hydraulic circuits and a prime mover. A first circuit includes a cylinder for driving an output member which may be connected to the polished rod, a first pump of the variable displacement, reversible flow type, together with a pair of fluid lines forming with the cylinder a closed-loop circuit. A controller is programmed to control the setting of the first pump so as to establish the velocity profile of the output member. A second hydraulic circuit is also in the form of a closed-loop containing first and second pumps, at least one of which is of the variable displacement type and is also controlled by the controller.