A walking beam pumping unit is provided for pumping liquid from wells having gas pressure therein. The unit is driven by gas pressure from the well reciprocating the piston of a pneumatic cylinder up and down to swing the walking beam correspondingly and pump the liquid from the well. Gas under pressure is directed from the wellhead through a two-way valve to the opposite ends of the hydraulic cylinder in alternating fashion, so the piston has power strokes in opposite directions. Each power stroke, besides moving the walking beam, serves to recompress the gas used for the preceding power stroke sufficiently to inject it into the sales line. The setting of the two-way valve is controlled by a pneumatic actuator supplied with gas under pressure from the wellhead and having a thimble valve responsive to the up and down movement of the walking beam by means of adjustable stops carried thereon. The horsehead is counter-balanced by weights or by a pneumatic cylinder attached to the horsehead and actuated by movement thereof to provide counter-balancing gas pressure. In one form of the invention, the counter-balancing gas pressure is stored in a hollow skid assembly.