A method and an arrangement for operating a process for extracting a fluid in a borehole are optimized. In the case of deep wells, the location of an interface depth in the borehole is detected. A pressure measurement of the pressure at the head of the borehole is made. The pressure in the liquid in the borehole below the interface depth is determined from the measured pressure at the head of the borehole and the detected location of the interface depth. The determination of this pressure is used for regulating the performance of an extracting device for the liquid that is to be extracted.