The effect of the leaf extract of Ginkgo biloba L. on platelet aggregation induced by oxidative stress was studied. The extract caused a dose-dependent inhibition of platelet aggregation stimulated with tert-butyl hydroperoxide (t-BHP) and Fe2+. Similar inhibitory activity was observed when platelets were exposed to H2O2 and Fe2+. Synergistic aggregation induced by a combination of t-BHP and Fe2+ or H2O2 and Fe2+ in association with suboptimal concentration of collagen or U46619, was prevented by the extract. However, the extract failed to inhibit aggregation in response to collagen, thrombin or U46619. Ginkgolides A, B and C inhibited platelet-activating factor-induced aggregation, but not oxidant-induced aggregation. These data suggest that the suppressive effect of the extract is specific on platelet aggregation stimulated by oxidative stress, and that this effect is involved in the mechanism related to its protective effect upon cerebral or myocardial injuries.