We previously reported that thrombin-induced Ca²⁺ mobilization was enhanced in aged rat platelets. Since Ca²⁺ mobilization in platelets is believed to be closely associated with platelet activation, we examined Ca²⁺ mobilization and the aggregatory response to ADP and thrombin in young (3 months) and aged (24 months) rat platelets. Blood levels of fibrinogen and Ca²⁺ in aged rats were higher than those in young rats. ADP-induced platelet aggregation in aged rats was significantly enhanced in platelet rich plasma and in washed platelet suspension, suggesting that age-associated hyperaggregability to ADP is attributable to changes in platelets themselves. On the other hand, thrombin (0.03-0.3 unit/ml)-induced aggregation in washed platelet suspension from aged rats was comparable to that from young rats. But, thrombin (0.3 unit/ml)-induced intracellular Ca²⁺mobilization was enhanced in aged rat platelets in the presence of extracellular Ca²⁺. Likewise, ADP-induced Ca²⁺ mobilization was enhanced in aged rat platelets. These results suggest that enhanced Ca²⁺ mobilization in aged rat platelets is associated with hyperaggregability to ADP but not to thrombin.