Before detailed studies of the physiology and pharmacology of central peptidergic neurons can be undertaken, the location of these neurons must be determined and the mechanism(s) by which they synthesize their peptide products must be explored. In the previous paper, Dr Hokfelt described his elegant immunohistochemical studies, which are designed to answer the questions: Where are peptidergic perikarya?; Where do these perikarya send their processes?; Do these processes branch extensively and innervate several structures?; and Do peptidergic cells contain more than one active product? By studying the effects of lesions on peptide levels in microdissected tissue samples, immunocytochemical data can be confirmed and extended. The microanalytical approach also allows one to determine the nature of the immunologically active substances in a tissue extract, and in vivo or in vitro pulse-chase studies provide the ultimate validation of immunohistological localization of peptidergic perikarya and new information about the biosynthesis of peptides and regulation of this biosynthetic process. Our recent studies of central proopiocortin- and neurophysin/vasopressin-producing neurons will illustrate the above points.