Some amphibians have the ability to form new limbs throughout their lives. The essential similarity between limb regeneration and the original development of the limb is that both involve the elaboration of new patterns of structures. While some investigators believe that the two developing systems use similar mechanisms to generate the limb pattern, others have stressed the basic differences in developing the regenerating limbs, and have concluded that different mechanisms exist. Both developing and regenerating urodele limbs can be caused to form supernumerary parts following tissue misalignment so that cells from normally disparate positions within the limb circumference confront each other (developing limbs; regenerating limbs, see ref. 11 for review). If similar mechanisms are operating, the developing limb should respond to grafts of regenerating limb tissue, and vice versa, by forming supernumerary outgrowths. If different mechanisms are operating, either the grafts should fail or the response should be disorganized. Our results with the axolotl Ambystoma mexicanum suggest that the patterning mechanisms are the same.