We have shown that [3H]mepyramine labels histamine H1-receptor-binding sites in bovine aortic membranes. Further characterization of H1-receptors in this tissue was done by the interaction of an unlabelled histamine receptor agonist or antagonist, with the radioantagonist [3H]mepyramine-binding sites. The competition-binding assays have uncovered differences in the characteristics of the agonist/receptor interaction not shared by antagonists. Agonists interact in the heterogeneous manner with the radioantagonist-labelled sites, showing shallow competition curves with the nH 0.50-0.72, whereas antagonists were devoid of this effect (steeper slopes of the inhibition curves nH approximately 1). The results suggest the presence in this tissue of multiple affinity states of histamine H1-receptor, differentiated by high and low affinity for agonists and the same affinity for antagonists.