INJURY1,2, nerves3 and hormones4 have been identified as essential for limb regeneration in amphibia. There seem to be differences in the ``current of injury'' between regenerating and non-regenerating types of amphibians5; partial limb regeneration can be induced in the latter type by simulating the ``current of injury'' of the regenerating form6. The cellular process of fracture healing in the amphibian is directly related to the electrical phenomena produced by the fractured bone7 and maximally effective ranges for current density can be determined at the cellular level. This led to the concept of a control system the key element of which was the induction of blastema formation in response to appropriate electrical factors8. The absence of regeneration in the mammal may therefore be due to the absence of adequate electrical factors. We report here the consequences of restoring factors which seem to be of practical as well as theoretical interest.