Washed human platelets were loaded with the Ca(2+)-sensitive photoprotein, aequorin, using hypoosmotic shock treatment-technique. Then aggregation and cytoplasmic ionized calcium concentration ([Ca2+]i) changes in response to collagen or thrombin were measured simultaneously in the aequorin-loaded human platelets with a Platelet Ionized Calcium Aggregometer. 764-3, an active component isolated from the Chinese medicinal herb Salvia Miltiorrhiza Bge, inhibited platelet [Ca2+]i rise as well as aggregation evoked by collagen or thrombin in the presence of extracellular Ca2+. After the extracellular Ca2+ was removed by addition of EGTA, collagen or thrombin, causing no aggregation, still elicited platelet [Ca2+]i rise which reflected Ca2+ mobilization from intraplatelet stores. Under this condition, 764-3 could also suppress platelet [Ca2+]i rise. Analysis shows that 764-3 inhibits platelet Ca2+ influx and Ca2+ mobilization with similar potency, which accounts for its suppression of platelet [Ca2+]i rise, and must contribute to its inhibition of platelet aggregation.