The brain functions as an information-processing center in which a variety of internal and external signals are decoded into defined molecular and cellular functions that often result in distinct behaviors appropriate for the environment. Although much has been learned about some of the specific neuronal activities supporting the transformation into a behavioral response, much remains to be understood about the interaction external stimuli and developmental processes that determine the onset and maintenance of stimulus perception and its association with a corresponding physiological response. Regenerative animals, such as newt and planarians, can regenerate their central nervous system (CNS), including the brain. The new findings about brain regeneration events after neuronal loss in regenerative animals may provide us an opportunity to clarify how to induce mammalian brain regeneration and how to conduct neural stem cell-transplantation. In addition, planarians are one of the most primitive CNS-possessing organisms. Planarians display behaviors responsive to signals coming from outside, and their nervous system can be regenerated and its function completely restored within a week because of their extra ordinal regenerative ability. In this study, we analyzed behaviors of clonal planarians with regenerated brain under different conditions, ie. normal light condition and completely dark condition. The results revealed that planarians regenerated under dark condition lost normal phototactic behavior, suggesting that light stimuli might be involved in the normal brain regeneration to produce normal phototactic behavior. Furthermore, we found that novel neuropeptide genes might be required for the proper functional recovery during brain regeneration. These results suggested that the neural peptides induced by neural activity of the newly regenerated neurons might be required for the proper phototactic behavior.