Abstract: Allium Cepa L. Var. agrogatum Don and Allium Macrostemon Bunge were shown to interfere with arachidonic acid metabolims of platelets. Cyclooxygenase products, thromboxane (TXA₂) and 12(S)-hydroxy-heptadecatrienoic acid (HHT) were decreased while PGE₂ did not change. However, the product of lipooxygenase hydroxy-eicosatetraenoic acid (HETE) was increased. Thus, the PGI₂/TXA₂ ratio was increased, a fact by which experimental atherosclerosis in rabbits was significantly inhibited by adminstration of Allium macrostemon extract. Our findings seem to be the theoretical basis for their utilization in preventing and treating thrombotic cardiovascular diseases.