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 (Hökfelt 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