[Ca²⁺]_i and pH_i were measured in unstimulated and thrombin-stimulated platelets from young (3 months) and aged (24 months) rats in the presence and absence of external Ca²⁺. There was no difference in the basal level of [Ca²⁺]_i between young and aged rat platelets in the presence of external Ca²⁺. On the other hand, thrombin-induced Ca²⁺ mobilization was markedly increased in aged rat platelets compared to young rat platelets in the presence of external Ca²⁺. This difference in Ca²⁺ mobilization was more pronounced in the absence of external Ca²⁺, suggesting that Ca²⁺ release from intracellular storages was enhanced in aged rat platelets. In contrast to what was observed for Ca²⁺, the basal level of pH_i in aged rat platelets was higher than that in young rat platelets whereas thrombin-induced pH_i increases (ΔpH) were similar in both groups in the presence and absence of external Ca²⁺. These results indicate that age-associated changes occur in thrombin-induced Ca²⁺ release from intracellular storages in rat platelets. These changes seem to be related to those in basal pH_i levels.