An implantable pump for pumping fluid to or from a hydraulic surgical implant inside a human's or an animal's body comprises a wall forming a chamber for the fluid, the wall including a base plate and a membrane, which is displaceable relative to the base plate to change the volume of the chamber to pump the fluid between the chamber and the surgical implant. The membrane is penetrable by an injection needle to add hydraulic fluid to or withdraw hydraulic fluid from the chamber, in order to calibrate the amount of fluid, and the membrane is self-sealing to seal the hole which is formed in the membrane by the penetrating injection needle. The implant typically is a hydraulic constriction device, which can be designed for treating reflux disease, urinary incontinence, impotence, anal incontinence or obesity.