For the purpose of elucidating the relationship between arteriosclerosis and platelet functions, platelet adhesiveness, aggregation, factor-3, (14)C-serotonin uptake and thrombodynamic function were measured in 92 patients with arteriosclerosis, including 29 cases of essential hypertention, 14 cases of chronic myocardial infarction, 29 cases of cerebral thrombosis and 11 cases of cerebral hemorrhage. As to the platelet functions of these arteriosclerotic diseases, a significant elevation of the platelet adhesiveness was found in cerebral thrombosis, chronic myocardial infarction, coronary insufficiency and essential hypertention. Platelet aggregation was elevated only in chronic myocardial infarction. The platelet factor-3 did not show any increased abnormalities in all cases but rather showed a lowering tendency in cerebral thrombosis. The (14)C-serotonin uptake was significantly decreased in cerebral thrombosis and hemorrhage. The thrombodynamic function revealed no abnormality. Considering the relation between the aging of arteriosclerotic patients and the platelet functions, platelet adhesiveness and aggregation were more enhanced in the group of over 51 years old than in the group of under 50 years old. The (14)C-serotonin uptake was more decreased in the latter group. Platelet factor-3 could not significant correlation to the aging. As to the arteriosclerotic lesion of the ocular fundus, platelet adhesiveness was more elevated in cases with the arteriosclerotic lesion such as Scheie Ⅰ, Ⅱ & Ⅲ degree and aggregation was observed to be accelerated only in cases with moderate arteriosclerotic lesion (Scheie Ⅰ & Ⅱ degree), compared with the normal group. But there was no significant correlation between the degree of progression of the fundus lesion and these platelet functions. Platelet factor-3 was found to be higher in the arteriosclerotic group than in the normal. No good relationship was noted between platelet (14)C-serotonin uptake function and the degree of the fundus lesion.