Two new peptides, methionine-enkephalin (met-enkephalin) and leucine–enkephalin (leu-enkephalin), have been isolated from brain, identified and synthesised1. The interest in these compounds lies in their morphine-like activity which was recognised and tested by their action on isolated preparations from the peripheral nervous system. Since morphine is used for its central action in producing analgesia, it is important to investigate whether these substances have activity in the central nervous system similar to that of morphine. We report here the findings of similar depressant actions of all three substances on brainstem neurones in the cat.