The lateral division of the central nucleus of the amygdala (CEAl) and the oval nucleus of the bed nucleus of the stria terminalis (BSTov) have been linked closely anatomically and functionally. To determine whether these regions may be subdivided further on a neurochemical basis, dual in situ hybridization was used to determine the colocalization of corticotropin-releasing hormone (CRH), enkephalin (ENK), or neurotensin (NT) with glutamic acid decarboxylase isoforms 65 and 67 [used concurrently as a marker for γ-aminobutyric acid GABA] in these nuclei. It was found that, for both regions, each peptide invariably was localized in a GABAergic cell. Although there was a similar overlap in the distribution of NT with ENK in the BSTov and CEAl, it was observed that CRH and ENK rarely were colocalized in either nucleus. To determine whether these distinct neuronal populations could be activated differentially, male rats were given a systemic injection of interleukin-1β (IL-1β; 5 μg/kg, i.p.), a stimulus that results in a robust increase in c-fos mRNA expression in the BSTov and CEAl. The neurochemical identity of these activated neurons showed striking similarities between the BSTov and the CEAl; All IL-1β-responsive cells were GABAergic, the majority of c-fos- positive cells expressed ENK mRNA (BSTov, 81%; CEAl, 94%), and some expressed NT mRNA (BSTov, 23%; CEAl, 22%), whereas very few expressed CRH mRNA (BSTov, 4%; CEAl, 1%). These data provide evidence for the existence of discrete neural circuits within the BSTov and CEAl, and the similarities in the patterns of neurochemical colocalization in these nuclei are consistent with the concept of an extended amygdala. Furthermore, these data indicate that intraperitoneal IL-1β recruits neurochemically distinct pathways within the BSTov and CEAl, and it is suggested that this differential activation may mediate specific aspects of immune, limbic, and/or autonomic processes. J. Comp. Neurol. 413:113–128, 1999. © 1999 Wiley-Liss, Inc.