1. The physical and chemical properties of substance P (extracted from bovine brain) and physalaemin were investigated by several methods. 2. During paper electrophoresis at pH 4.95 substance P migrates towards the cathode, physalaemin remains at the starting point. 3. During carrier-free glucose gradient electrophoresis substance P moves to the cathode whereas physalaemin remains at the starting point. 4. Electrofocusing of physalaemin reveals an isoelectric point of pH 7.0 whereas substance P has an isoelectric point of pH 10.0 to 10.5. 5. The biological activity of physalaemin was reduced by 70% after treatment with diazotized p-nitraniline, but that of substance P and eledoisin was not reduced. All three peptides were inactivated by treatment with phenylisocyanate. It was concluded therefore, that phenolic hydroxyl groups are not essential for the biological activity of substance P.