The photoaffinity probe [125I]iodoazidophenpyramine was used to label irreversibly the H1-receptor in membranes of several guinea pig brain regions and of the cerebral cortex of the rat, mouse, and pig. Following sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, two main bands were specifically labeled in all tissues: a 56-kilodalton (kDa) peptide and a 41-47-kDa peptide whose relative importance diminished in the presence of protease inhibitors. This indicates that, in all tissues examined, in spite of evidence for pharmacological heterogeneity, the ligand recognition domain of the H1-receptor resides in a 56-kDa peptide.