By studying the action of alkylposphates on the isolated frog heart (the Straub-Fuhner method) and on the frog heart in vivo (Engelmann's method), the author obtained the following results: 1. The auriculo-ventricular block occurs whan exposed to Parathion at a low concentration. 2. In the case of Parathion at a higher concentration there occurs a decrease in the amplitude of heart-beat, and further on brings about the diastolic cessation. 3. Such changes as mentioned above do not return to normal unless properly treated. 4. Atropine and interceptors of the autonomic nerves can well act antagonistically to the action of Parathion. 5. Epinephrine acts antagonistically against Parathion. 6. Acetylcholine and ergotamine act as to invigorate the action of Parathion. 7. Diazinon and Chlorthion act more or less in the same manner as Parathion.