Bombesin is a gastrointestinal hormone and neurally active peptide. It stimulates gastric acid secretion via gastrin release. The peptide is behaviorally active in suppressing food intake and lowers body temperature. The mammalian endogenous form of the peptide is ectopically produced in small cell carcinoma of the lung. These tumor cells also have bombesin receptors. Therefore, a possible means of chemotherapy could consist of drugs derived from this peptide. The synthesis of a series of analogues is proposed. One group is composed of agonist peptides designed for enhanced potency and efficacy. The peptides contain a minimum active sequence with changes to stabilize the conformation. Included in this group is a cyclic analogue. In the other group are possible specific antagonists. The peptides will be screened for activity and antagonism in chemotaxis to macrophages and tumor cells and in food intake behavior.