Data were presented showing that I1-imidazoline sites show a unique ligand specificity that differs markedly from that of any of the alpha 2-adrenergic subtypes or the I2-imidazoline sites labeled by [3H]idazoxan. On the other hand, the ligand specificity of I1-imidazoline sites is maintained across mammalian species (cow, rat, dog, and human) and between different tissues and cell types. I1-Imidazoline sites can be further distinguished from I2 sites because the latter, unlike I1 sites, were not present in RVLM membranes from bovine brain stem. Furthermore, I1-imidazoline sites were modulated by guanine nucleotides with a specificity appropriate for a receptor coupled to G-protein and were mainly localized to plasma membranes. I1-Imidazoline sites show a unique pattern of distribution between diverse tissues and cell types and appear to be a neuroepithelial marker as well as being present in secretory cells of the pancreatic islets. The widespread distribution of I1-imidazoline sites implies that the functional significance of this putative receptor may have been underestimated. The signaling pathway associated with the I1-imidazoline receptor remains to be fully elucidated, but is likely that activation of phospholipase A2 leading to release of arachidonic acid and subsequent generation of prostaglandins plays a major role.