The neuropeptide Y (NPY) Y5 receptor is believed to be involved in the central regulation of appetite. Thus, antagonists of this receptor have been pursued as potential therapeutic agents for the treatment of obesity. A novel series of potent and selective phenylamide or biaryl urea NPY Y5 receptor antagonists was identified. Four representative compounds from this series, SCH 208639 (N-[4-[(1,1-dimethylbutyl)thio]phenyl]-2,2-dimethylpropanamide), SCH 430765 (N-[[[3′-fluoro[1,1′-biphenyl]-4-yl]amino]carbonyl]-N-methyl-1-(methylsulfonyl)-4-piperidinamine), SCH 488106 (N-[[[3′,5′-difluoro[1,1′-biphenyl]-4-yl]amino]carbonyl]-N-methyl-1-[(5-methyl-3-pyridinyl)carbonyl]-4-piperidinamine) and SCH 500946 (N-[[[5-(3,5-difluorophenyl)-2-pyrazinyl]amino]carbonyl]-N-methyl-1-(methylsulfonyl)-4-piperidinamine), behaved as competitive antagonists in radioligand binding assays, but displayed apparently insurmountable antagonism in a cell-based functional assay. The apparently insurmountable antagonism was due to slow receptor dissociation rates rather than covalent binding, because the antagonists' effects could be reduced by extensive washing of cells after antagonist exposure. A novel radioligand, [35S]SCH 500946, was also developed and used to characterize the interaction of these antagonists with the NPY Y5 receptor. [35S]SCH 500946 had high affinity for the NPY Y5 receptor (Kd = 0.29 nM), and the binding kinetics (kon 4.414 × 107 M−1 min− 1; koff 0.009816 min− 1) confirmed that the compound slowly dissociates from the receptor. In a competition binding assay, NPY failed to displace [35S]SCH 500946 completely, indicating that the binding sites for NPY and [35S]SCH 500946 are not identical. These data indicate that the apparent insurmountable antagonism of these NPY Y5 receptor antagonists is attributable both to slow receptor dissociation rates and to binding at a site distinct from NPY.