Lysergic acid diethylamide (LSD) potentiated the response of guinea‐pig ileum to substance P but not to histamine. It also inhibited the disappearance of substance P when incubated with guinea‐pig brain extract but not when incubated with chymotrypsin. Eserine, morphine, mescaline, chlorpromazine, ergometrine, strychnine and 2 bromo‐LSD did not have this effect. Oxytocin was not destroyed by...