[Ca2+]iand pHiwere 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+]ibetween 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 pHiin aged rat platelets was higher than that in young rat platelets whereas thrombin-induced pHiincreases (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 pHilevels.