The effect of Panaquilon (Partial Component of Panax ginseng), on the isolated clam heart, the motility of the isolated clam heart suspended in physiological solution for clam heart was recorded by the Magnus Method. 1. The concentration of 5 X 10<sup>-6</sup> of Panaquilon produced cardiac arrest in the diastolic state, which resumed the normal state slowly after changing the physiological solution for clam heart. 2. Cardiac arresting action of Panaquilon was antagonistic when pretreated with Serotonin.