In this study, the platelet intracellular calcium and serotonin (5-HT) concentrations as well as the hemorheological properties were compared in 65 hypertensive (HT) and 25 age-matched normotensive (NT) subjects. The result showed that the platelet intracellular calcium concentration in the HT group was significantly higher than in the NT group (165.4 ± 12.6 vs. 118.9 ± 12.4 nM, p < 0.05). However, the platelet 5-HT content in the HT group was significantly lower than in the NT group (0.28 ± 0.03 vs. 0.38 ± 0.03 nmol/10<sup>8</sup> platelets, p < 0.05). Moreover, red cell flexibility, red cell deformability, hematocrit, fibrinogen concentration and plasma viscosity were all significantly increased in HT subjects. The whole blood viscosity was generally higher in the HT group. In conclusion, the higher platelet intracellular calcium and lower 5-HT level as well as abnormality of hemorheological properties may be important factors in the initiation and perpetuation of hypertension.