As part of a programme to develop fluorescence-based methods for the study of the interactions between G-protein coupled receptors (GPCRs) and their ligands the preparation of low molecular weight fluorescence-labelled neuropeptide Y (NPY) Y(5) antagonists is reported. The naphthylsulfonyl group in the potent quinazoline-type NPY Y(5) receptor antagonist CGP 71683A was replaced with a dansyl, nitrobenzoxadiazole (NBD) or acridine-9-carbonyl group. In radioligand binding studies on human Y(5) receptor expressing HEC-1B cells the substances labelled with acridine (K(i) 311 nM) and NBD (K(i) > 1000 nM) proved to be moderately active or inactive, respectively. By contrast, a K(i) value of 49 nM was found for the dansyl analogue compared to 2 nM for CGP 71683A. No binding to Y(1) receptors (SK-N-MC cells, displacement of [(3)H]propionyl-NPY) was detected for the new compounds at concentrations </= 1 microM.