Problems on the relationships of lipid metabolism, blood coagulability and arteriosclerosis are now attraction a great interest of many investigators. As a link in the series of studies along this line, we have been carrying on investigations concerned with lipid metabolism and blood coagulation, fibrinolysis and platelets for the past several years. In the present experiments we studied the lipid metabolism of platelets, particularly about the lipid synthesis and various functions of platelets, using 14C-acetate. Namely, we attempted to compare the functions and the lipid synthesis of platelets, such as the adhesive rate, the retraction rate, the factor-3, and the serotonin uptake of platelets in the arteriosclerosis syndrome such as coronary insufficiency and cereberal-malacia, and in diabetes and malignant tumors. As a result we have found that of the arteriosclerosis syndrome those with increase in the levels of the total lipid and phospholipid, and an increase of the capacity of NEFA synthesis of platelets show a marked acceleration of the adhesive rate. In addition, the factor- 3 and the rate of serotonin uptake have been found to be abnormally low in those groups with decreased capacity of total lipid synthesis. In the case of diabetes and malignant tumor groups there can be recognized not any definite trend, and the platelet retraction is not at all affected by changes occurring in the lipid synthesis in all these three groups. In other words, these findings seem to indicate that there is a mutual relationship between lipids and platelet functions in the platelet functions in the platelets of arteriosclerosis diseases.