Rats (Sprague Dawley, male, 200 g) were fed with 15% corn oil containing a large quantity of 18 2 (linoleic acid) for 3 weeks, and were followed by feeding the petroleum ether extracts from Korean red ginseng for 3 weeks. cGMP was produced more in platelets prepared from both 15% corn oil and petroleum ether extracts-fed group than in platelets only 15% corn oil-fed group, indicating that the production of cGMP is increased by feeding the petroleum ether extracts. When this platelet was stimulated by phorbol-12-myristate-13-acetate (PMA), the level of cGMP was decreased. However, the platelets in medium containing protein fraction (200 $mu extrm{g}$/ml) was stimulated by PMA, the production of cGMP inhibited by PMA was increased by 3 times or more. These results suggest that both the protein fraction and the petroleum ether extracts from Korean red ginseng are synergistic in the productiorl of cGMP, and they may have the antiplatelet effects. Key words Linoleic acid, cGMP, lipophilic and protein fractions, red ginseng, rat platelets