The effects of ginseng saponins on platelet aggregation, cyclic AMP and cyclic GMP level in platelets were studied. Ginseng saponins was found to be a potent inhibitor of platelet aggregation induced by arachidonic acid ( AA ) , ADP and thrombin in vitro and in vivo. The concentrations of ginseng saponins producing 50% inhibition of platelet aggregation induced by 0.55HM AA, 1.8-3.5uM ADP and 0.6-0.7 NIH u/ml of thrombin were 0.44, 1.20 and 1.32 mg/ml respectively. The inhibition % of aggregation induced by AA and ADP in vivo from 2-30min after iv administration of ginseng saponins ( 80 mg/kg) were 99-30.44 and 80.7-40.7, respectively. Also, cyclic AMP and cyclic GMP level in platelets were determined. It was observed that ginseng saponins significantly increased cyclic AMP level in platelets in a dose-dependent manner. But ginseng saponins did not affect cyclic GMP level in platelets. Our results indicate that the inhibition of ginseng saponins on platelet aggregation may be associated with an increase in cyclic AMP level.