Research efforts center on documenting the organization and variation in the nervous systems of living vertebrates and utilizing these data to elucidate mechanisms underlying brain evolution. We plan to examine the peripheral distribution and innervation of electroreceptive ampullary organs and mechanoreceptive neuromasts, their embryonic development, and the ascending neural pathways mediating these sensory modalities of the lateral line system in axolotls, Ambystoma mexicanum, a group of permanently aquatic salamanders. These studies will involve a number of microdissection and surgical approaches on embryos, larvae, juveniles and adults, primarily with the use of horseradish peroxidase as a marker of embryonic tissues and neural pathways. Axolotls will be examined in this study because they are the only group of vertebrates that retain a primitive pattern of electroreceptors whose embryology can be easily studied. The data generated by this study will be used to begin an extensive analysis of a rather unique natural experiment: the loss and re-evolution of electroreception. Electroreceptors were lost early in the evolution of bony fishes, and they re-evolved several times independently. There is, thus, the rare opportunity to examine effects on the brain of the loss of a major sensory system and to recognize possible constraints on the ways in which electroreceptive systems can evolve.