In an effort to develop potent antiplatelet agents, 12 O-prenylated (2-13) and 10 O-allylated (14-23) chalcones were synthesized and screened for in vitro inhibitory effects on aggregation of washed rabbit platelets induced by ADP (20 μM) and collagen (10 μg/mL). In addition, the platelet aggregation activity of previously synthesized Mannich bases of heterocyclic chalcones (MBHC) (24-62) was evaluated. The preliminary structure-activity relationships suggested that the antiplatelet activity was governed to a great extent by the presence of a pyridyl ring-B and a hydroxy group at position C-3' in ring-A of the MBHC templates.