Neuropeptides play significant roles in chemical neurotransmission, and their involvement has greatly increased the number of identified transmitters. This chapter describes the application of the techniques of Drosophila genetics for manipulating the expression of a specific neuropeptide gene and the developmental mechanisms by which this gene is normally expressed in a stereotyped set of unrelated neurons. The chapter explores the logic of these developmental mechanisms in the context of the physiologically mature nervous system by creating and examining three abnormal patterns of expression. The results indicate that analysis of the FMRFamide neuropeptide gene in Drosophila give hope that it will represent a valuable model system with which to address basic questions regarding the biological functions of neuropeptides using a molecular genetic approach. Drosophila preproFMRFamide contains multiple FMRFamide-like neuropeptides that reflect as many as seven different sequence variations in the amino-terminal portions of the molecules. However, whether such variety indicates differences in function for different peptides is not known.