It was previously reported from our laboratory that the rate of deterioration of the force of contraction was slower in heart from Panax ginseng extract treated rats. The study carried out to elucidate its mechanism of the action on hearts. The cyclic AMP content in the rat hearts was measured by the method of radioimmunoassay techniques. Panax ginseng extract (100mg/kg/day) was administered orally to male Sprague-Dawley rats weighing 150g to 250g for 1 week and after 24 hrs the hearts were isolated and the cyclic AMP content in the fresh heart was assayed. The difference in cyclic AMP content between the rats treated with Panax ginseng extracts and normal rats was not significant. Panax ginseng extract(l00mg/kg/day) was administered orally to the rats for I week and after 24 hrs the hearts were isolated and perfused with Krebs-Henseleit buffer (pH7.4) for 90min. The cyclic AMP content in the both treated and normal rats was not also significantly different. On the other hand, when total ginseng saponin (50mg/kg/day) was administered orally to rats for 1 week and after 24 hrs, the isolated hearts were perfused with Krebs Henseleit solution for 32min, the cyclic AMP content in total ginseng soponin treated hearts was decreased by 18.7% compared to normal rats. It was also observed that when isolated hearts were perfused with total ginseng saponin $(10^{-4}g/ml)$ for 12 min after 30 min equilibration period, the cyclic AMP content in total ginseng saponin treated hearts was decreased by 23.7% compared to normal rats. Isolated hearts were perfused with ginseng saponins $(10^{-4}g/ml)$ or with Krebs-Henseleit solution alone for 10min and subsequently with dl-isoproterenol $(1/2{ imes}10^{-6}M)$ until the positive inotropic effect of isoproterenol was initiated. The cyclic AMP content in each rat hearts treated with total ginseng saponin, or with ginsenoside $Rb_1$, or with Krebs-Henseleit solution alone were increased by 35.5%, 42.4%, 47.5%, respectively, compared to normal rats.