The functional activity and selectivity of the novel 5-HT1D receptor antagonist GR 127,935 (2'-methyl-4'(5-methyl-1,2,4 oxadiazol-3-yl)-biphenyl-4-carboxylic acid [4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-amide) was investigated at cloned human 5-HT1A, 5-HT1D alpha, 5-HT1D beta and opossum kidney (OK) 5-HT1B receptor sites. 5-HT1 receptor-mediated activity was studied by measuring the inhibition of forskolin-induced cAMP formation in cell lines expressing these receptors (Bmax (fmol/mg protein): human epitheloid carcinoma HeLa/5-HT1A: 1285, OK/5-HT1B: 52, Chinese hamster ovary CHO-K1/5-HT1D alpha: 181 and CHO-K1/5-HT1D beta: 685). GR 127,935 did not show 5-HT1D beta receptor-mediated agonist activity in permanently transfected CHO-K1 cells, whereas at submicromolar and higher concentrations intrinsic agonist activity was observed in HeLa/5-HT1A,OK/5-HT1B and CHO-K1/5-HT1D alpha cells. GR 127,935 showed potent (KB value: 1.3 nM) and silent antagonism at CHO-K1/5-HT1D beta receptor sites. The antagonist activity of 1 microM of GR 127,935 at CHO-K1/5-HT1D alpha and OK/5-HT1B receptor sites was only partial and less pronounced. This contrasts with the silent antagonism of methiothepin at the 5-HT1D alpha (KB value = 11.8 nM), 5-HT1D beta (KB value = 6.9 nM) and 5-HT1B (KB value = 49.3 nM) receptor subtypes. GR 127,935, when tested at 10 microM, was found to be a weak and partial antagonist of HeLa/5-HT1A receptors.(ABSTRACT TRUNCATED AT 250 WORDS)