A hydraulic drive system for driving component parts such as harvesting units of a cotton harvester head assembly. The hydraulic drive system includes a source of actuating fluid and fluid pressure actuators for driving the harvesting units independently of each other. Suitable conduit structure connects each fluid pressure actuator to the source of actuating fluid. The conduit structure is preferably flexible to facilitate transverse positioning of the harvesting units on the head assembly. A control valve assembly is connected to the conduit structure and is operably interposed between the source of actuating fluid and the fluid pressure actuators for selectively interrupting and effectuating the supply of actuating fluid from the source of actuating fluid and each of the fluid pressure actuators thereby influencing operation of the harvesting units.