Platelet aggregability was compared between platelets isolated from normal subjects and patients with diabetes mellitus in order to evaluate the effects of calcium channel blockers and insulin on the platelet function. The threshold aggregating concentration of adenosine diphosphate (ADP), which induces the second phase aggregation and reflects the platelet release reaction, was found to be significantly lower in diabetics than in normal subjects (1.8 µM vs 7.5 µM ). It was observed that the second phase aggregation curve induced by ADP was inhibited by in vitro treatment of platelets with insulin (10-100 µU/ml), verapamil (1-10 µM), and diltiazem (1 µM) in diabetics. The result also shows that the inhibition was enhanced when insulin and calcium channel blockers were used together for in vitro treatment of diabetic platelets. Thus, the present study suggests that the use of calcium channel blockers combined with insulin would be more effective than the use of insulin alone in the prevention of diabetic vascular disease.