Time-dependent changes in platelet aggregation in whole blood, platelet-rich plasma (PRP) and washed platelets were studied in streptozotocin-induced diabetic rats. Collagen-induced aggregation of whole blood and PRP from diabetic rats were significantly reduced within 8 weeks after induction of diabetes, although that in washed platelets were increased from 8 weeks. Plasma from diabetic rats within 8 weeks attenuated platelet aggregation, whereas diabetic plasma at 12 weeks showed no inhibitory effect. Insulin treatment normalized aggregation in whole blood and PRP and abolished the antiaggregatory activity of diabetic plasma. These results suggest the plasma antiaggregating activity appears in the early stage of diabetes, which may contribute to the hypoaggregation in whole blood and PRP. The inhibitory activity disappeared in the later stage. Plasma factor(s) accounting for the antiaggregatory effect of diabetic plasma has not yet characterized.