An implantable system for monitoring vital parameters via bio-sensors inside freely moving laboratory animals and its powering system are presented. The required 2 mW are harvested by the magnetic coupling with an external coil placed under the living space of the animal. The servo X-Y rails move the external coil and track the animal. Dynamic power-adaptation keeps the harvested power level constant against misalignments of transmitting coil and moving animal. Entire system and basic blocks integrated using a 0.18 um CMOS technology are presented. Experimental results show the effectiveness of the powering system.