The ability of the human neuropeptide YY 1 receptor subtype to increase the extracellular acidification rate in different cell lines was investigated by using the Cytosensor Microphysiometer. In CHO-Y 1 cells (Chinese Hamster Ovary cells expressing the cloned human neuropeptide YY 1 receptor), neuropeptide Y increased the acidification rate by up to 15% of the basal level with a -Log(EC 5 0 ) of 7.42. As expected for neuropeptide YY 1 receptors, this response was potently inhibited by the neuropeptide YY 1 -selective non-peptide antagonist BIBP3226 ((R)-N 2 -(diphenylacetyl)-N-[(4-hydroxy-phenyl)methyl]-d-arginine amide). Its enantiomer BIBP3435 ((S)-N 2 -(diphenylacetyl)-N-[(4-hydroxy-phenyl)methyl]-d-argininamide) was less potent. The antagonists themselves did not affect the extracellular acidification rate at concentrations up to 10 μM. In SK-N-MC cells (a neuroblastoma cell line of human origin that expresses the neuropeptide YY 1 receptor) no change of the acidification rate could be observed in the presence of neuropeptide Y at concentrations up to 1 μM. For control, the neuropeptide YY 1 receptors were also investigated by assessing whole cell radioligand binding and, at the functional level, by assessing their ability to decrease the forskolin-induced accumulation of cAMP. The specific (i.e., neuropeptide Y-displaceable) binding of [ 3 H]neuropeptide Y was to a homogenous class of high-affinity sites in both SK-N-MC and CHO-Y 1 cells. The equilibrium dissociation constants for [ 3 H]neuropeptide Y, the total number of binding sites and the kinetic constants for association and for dissociation were similar. Neuropeptide Y produced a dose-dependent inhibition of forskolin-induced cAMP accumulation in SK-N-MC cells (-log(EC 5 0 )=9.40) but it did not affect cAMP accumulation in CHO-Y 1 cells. Non-transfected CHO-K1 cells were used as negative control throughout the study. No binding or response could be observed in these cells. Our data suggest that the signalling mechanisms of neuropeptide YY 1 receptors are closely related to the cell type in which they are expressed.