gamma-Aminobutyric acid (GABA) was detected immunocytochemically in the rat hippocampal formation utilizing a new antiserum made against GABA conjugated to bovine serum albumin. GABA immunoreactivity was found to be concentrated in pericellular networks of fibers around both dentate granule and hippocampal pyramidal neurons. Ultrastructural analysis indicated that GABA-immunoreactive (IR) terminals most often contained small clear vesicles and made symmetrical contacts with somata and dendritic shafts of hippocampal neurons. In addition, numerous GABA-IR neurons resembled basket interneurons that have been described in previous reports.