Regulation of gene expression has been involved in long-term memory consolidation. Present results support the role of Rel/NFκ-B like activation in this process. In the crab Chasmagnathus, the spaced presentation of 15 or more danger stimuli induces long-term habituation (LTH), while no LTH is observed after a massed training of 600 trials. When a group trained with 30 spaced trials was compared with a passive control group and massed trained groups, a higher level of specific Rel/κ-B like DNA-binding activity was found in brain nuclear extracts. These results strongly suggest that the enhancement of Rel/κ-B like DNA-binding activity in the brain is specifically related to LTH formation.