In this study, the binding of [H-3]5-HT to the cloned dog 5-hydroxytryptamine(1B) (dog 5-HT1B) receptor, stably expressed in Chinese hamster ovary cells (ATCC CCL 61) (CHO-K1), was characterised and its pharmacology compared with that of the cloned human and rat 5-MT1B receptors. [H-3]5-HT specifically labeled, with high affinity, an apparently homogeneous population of binding sites in the dog 5-HT1B receptor cell line yielding a pK(d) of 8.1. [H-3]5-HT inhibition and agonist-induced [S-35] guanosine 5'[gamma-thio] triphosphate ([S-35]GTP gamma S) binding studies revealed comparable results with the human but not the rat 5-HT1B receptor. In all three recombinant receptor cell lines, methiothepin displayed inverse agonism and GR127935 (N-[4-methoxy-3-(4-methyl-1-piperizinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazole-3-yl)[1,1'-biphenyl]-carboxamide) weak partial agonism. (C) 1998 Elsevier Science B.V. All rights reserved.