The investigation is concerned with the action of ginseng saponin on the contractile force in the rat heart and with the elucidation of the mechanism of the action. The effect of total ginseng saponin, ginsenoside Rb$_{1}$ of protopanaxadiol derivatives and ginsenoside Re of protopanaxatriol derivatives on the contractile force in isolated spontaneously beating normal rat heart was investigated. Total ginseng saponin was obtained from white ginseng by the method of Shibata and Namba. Ginsenoside Rb$_{1}$ and ginsenoside Re were isolated by the method of and Han, respectively. Total ginseng saponin exhibited a slight increase of the contractile force. Ginsenoside Rb$_{1}$ increased markedly the contractile force and dose dependent increase in contractile force was observed. However, ginsenoside Re did not increase the contractile force, but it prevented spontaneous decrease of the contractility of the heart. The mixture of the same dose of ginsenoside Rb$_{1}$ and Re showed a slight increase in the contractile force and its effect was similar to that obtained by total ginseng saponin. Pretreatment with propranolol abolished the positive inotropic effect of ginsenoside Rb$_{1}$ and the positive inotropic effect of ginsenoside Rb$_{1}$ was not observed in a reserpinized rat heart. Pretreatment with ginsenoside Re decreased or abolished the positive inotropic effect of epinephrine. Activities of Na+, K+ -ATPase were inhibited by ginsenoside Rb$_{1}$, total ginseng saponin and ginsenoside Re and these inhibitory effects were dose dependent. The results suggest that catecholamine release or inhibition of Na+, K+ -ATPase activities may be involved in the positive inotropic effect of gindenoside Rb$_{1}$. Ginsenoside Re counteracted the positive inotropic effect of ginsenoside Rb$_{1}$.