We have studied the binding of [3H]-NPY and the newly developed non-peptide Y1 receptor antagonist [3H]-BIBP3226 to intact SK-N-MC cells and CHO-K1 cells transfected with the human NPY Y1 receptor gene i.e. CHO-Y1 cells. Whereas the association and dissociation of the specific [3H]-NPY binding was slow, the binding kinetics of [3H]-BIBP3226 binding was very rapid. Saturation binding of both radioligands reveal the presence of an apparently homogeneous population of high affinity binding sites in both cell lines. The corresponding equilibrium dissociation constants are similar for the two cell lines and are close to those obtained from previous competition binding experiments. The specific binding of both radioligands was completely and with high affinity displaced by BIBP3226 and its inactive (S)-enantiomer BIBP3435 was much less potent. Whilst the NPY Y1 agonists NPY, PYY and [Leu31-Pro34]-NPY completely and potently displaced [3H]-NPY binding, they could only displace 70 to 80% of the [3H]-BIBP3226 binding sites in CHO-Y1 and SK-N-MC cells. A possible explanation can be that only part of the receptors are G-protein coupled. In agreement pertussis toxin was found to reduce high affinity [3H]-NPY binding sites in CHO-Y1 cells whereas [3H]-BIBP3226 binding parameters remained unchanged.