The physiological role of substance P (SP) is still largely a matter of conjecture. Its extremely high potency in pharmacological tests (stimulation of gut, depression of blood pressure, and vasodilation), its almost exclusive occurrence in the gut and certain areas of the brain (Pernow 1953; Lembeck and Zetler, 1962; McLennan, 1963), and its association with the subcellular particles containing nerve endings (Euler and Lishajko, 1961; Euler 1963; Ryall 1964), suggest an important role in nervous activity. However, this is a suggestion that can be substantiated only after the isolation of a pure chemical entity in sufficient quantity for characterization and pharmacological study. The pure material may after all not show all the biological properties heretofore ascribed to substance P.