Oleanolic acid (3beta-hydroxy-olea-12-en-28-oic), a pentacyclic triterpenoid, exists widely in the plant kingdom and has a wide variety of pharmacological effects such as antitumor, antifungal, insecticidal, hepatoprotective and anti-HIV activities. This paper reports that oleanolic acid induces the aggregation of rabbit platelets, a mechanism was also investigated. Oleanolic acid at concentrations of 25, 50, 100 and 200 microM induced the aggregation of washed rabbit platelets in a concentration-dependent manner. Pretreating the platelets with U73122, a phospholipase C (PLC) inhibitor, blocked the oleanolic acid induced-aggregation, whereas acetylsalicylic acid (ASA), a cyclooxygenase (COX) inhibitor, had no effect. In addition, the effect of oleanolic acid on serotonin secretion, which is a marker for dense granule secretion, was determined. Oleanolic acid induced serotonin secretion in a similar concentration-dependent manner as observed with platelet aggregation. Pretreating the platelets with U73122 blocked the oleanolic acid-induced serotonin secretion and cytosolic calcium mobilization. Overall, these results suggest that oleanolic acid can induce platelet aggregation, which may be mediated by the stimulation of PLC-mediated cytosolic calcium mobilization.