1. The auther studied the interrelationship between the sensibility of heart of fishes to neuro-hormones (epinephrine, norepinephrine and acetylcholinie) and 5-hydroxytryptamine and the differentiation of nervous system in the heart . Attempts were also made to develop the similarity and systematic relationship of inter-genus and inter-species. 2. Judging from the reactions to neurohormones, it is considered that inter-genus of locah (Misgurnus mizolepis GNTHER) and eel (Anguilla japonic TEMMINEK & SCHJEGEL) are more closely related than that of Crucian (Carassius carassius L. ). 3. As the sensibility of locach to 50 jhdroxytryptamine is more sensitive over 10, 000 times than that of a Mollusca, Meretrix lusoria RODING, it is very excellent material for the bioassay of 5-jydroxytryptamine.