The development of an active implantable device for measuring electrocardiogram (ECG) is presented. The study is a part of a project which aims at developing implantable ECG instrumentation with wireless data and power transfer ( http://www.ele.tut.fi/tule ). The developed implant presented here has all the measurement electronics as well as power and data communication instrumentation included. The implant itself contains no battery, while power for the implant is transferred electromagnetically from an external reader device. The results of testing the implant attached on the body surface and in vitro in a water container are also presented. The developed system was also successfully tested in in vivo measurements, which were conducted on four cows with an implantation time of 24 h. The in vivo testing of implant in cows was conducted by a veterinarian in supervised conditions under approved animal experiment licence.