During the course of the last decade a considerable number of biologically active peptides has been identified in the nervous system; several of these recently discovered peptides fulfill the criteria for a neurotransmitter function (H�kfelt et al., 1980a) ; Snyder, 1980 ; Bloom, 1981 ; Krieger and Martin, 1981a , b ; Krieger, 1983 ; Krieger et al., 1983 . This remarkable expansion in the vocabulary of neurotransmission has generated questions that can be addressed, at least in part, with biochemical approaches.