Nitricoxide (NO) has been reported to play various roles as a signal transmitter. However, detailed functions of NO have yet to be clarified. We have developed a fluorescent indicator for NO imaging in living cells. The N-nitrosation of newly designed and synthesized 4-((3-amino-2-naphthyl)aminomethyl)benzoic acid (DAN-1) by NO yielded the highly fluorescent triazole-form. The membrane permeable ester