There is controversy as to whether platelet intracellular free calcium ([Ca2+],) is increased in spontaneously hypertensive rats (SHR) as compared with Wistar-Kyoto (WKY) rats. Discrepant results may be due to methodological problems including platelet activation during the collection process and leakage of intracellular dye used for [Ca], measurement To provide further insight into this problem, [Ca2+], was estimated in fura-2-loaded platelets isolated from eight SHR and seven WKY rats at 12–14 weeks of age by using a two-syringe blood collection method and a correction method for fura-2 leakage. Basal [Ca2+], was higher in SHR than in WKY rats (61.6±5.6 vs. 54.0±3.9 nM,p< 0.02). However, the difference disappeared when a correction for fura-2 leakage was not used (109.7±18.4 vs. 94.9±9J nM, P>0.1). Thus, differences in [Ca2+], between SHR and WKY rats may be obscured if dye leakage from platelets is not taken into account Thrombin (0.1 units/ml) induced a rise in [Ca1+]t that was greater in SHR than WKY rats, both in the presence (491.4±31.6 vs. 377.5±21.7 nM,p< 0.002) and absence (264.9±33.6 vs. 228.2±30.1 nM,p< 0.05) of calcium in the media. These results indicate that thrombin-stimulated calcium influx as well as discharge of calcium from intracellular stores is increased in SHR platelets. Thus, under both basal and stimulated conditions, platelet calcium handling is abnormal in the SHR.