This study was undertaken to verify if a fluorescent derivative of NRB could furnish information about the cellular target/s involved in the selective rat-specific and tissue-selective action of unlabeled NRB. To this end, we attached the NBD fluorophore to the NRB molecule. The NBD group is known to bleach very rapidly in respect to other commercially available fluorophores, nevertheless we decided to use NBD because of its relatively small size, in the attempt to minimize the risk of inducing a distortion of the chemical-physical properties of NRB. We compared the fluorescence distribution of NBD-NRB (NRB-AF12) observed in NRB-sensitive cells (freshly isolated rat caudal artery myocytes) with that observed in NRB-insensitive cells (several cultured cell lines). Preliminary experiments conducted in our lab indicated that NRB-AF12 has the same species- and tissue-specific contractile properties of unlabeled NRB (manuscript in preparation), and therefore is a reliable tool for the purpose of this work.