Abstract: Apparatus for pumping a liquid from a well comprises a drive unit on the surface, a well-bottom pump unit, two supply lines interconnecting the drive unit and the pump unit and a liquid discharge line extending from the pump unit to the surface for discharge of liquid. The pump unit has a drive piston which is reciprocated within a drive cylinder by applying pressure alternately by means of a changeover valve to the two supply lines. The drive piston drives a double acting driven piston which pumps oil from a well. The changeover valve is controlled by a logic unit which receives signals from sensors measuring the pressure and/or flow rate in a return line.