By using immune sera which recognize one or more of the 3 peptides, somatostatin-14 (SS14), somatostatin-28 (SS28) and somatostatin-28(1-12) (SS28(1-12)) we have characterized their immunohistochemical distribution in the hippocampal formation. There exist at least two independent neuronal systems containing pro-somatostatin-related peptides: an intrinsic system of cells in the polymorphic layers which branch locally, and a dense terminal field in the molecular layer of the dentate gyrus that may constitute a portion of the entorhinal-dentate projection. In addition, SS28 is the dominant form present in cell bodies, whereas SS28(1-12) is preferentially localized in neuronal processes and terminals. We could not detect SS14 immunohistochemically.