The immunohistochemical visualization of peptide-containing neurons forms the basis of most chapters in Volume 4. The volume deals with the structural organization of neuropeptide-containing neuronal systems in the CNS as revealed primarily by the immunohistochemical technique. The volume also includes a major chapter on GABA neurons in the CNS as revealed by an antiserum against GAD. This chapter gives a unique overview of putative GABA-ergic neurons in the CNS and contains a wealth of previously unpublished material, including the first complete mapping of GAD in the rat CNS. Thus, this volume supplements and equals the well-received information in the first three volumes in the series.