To investigate shear stress-induced platelet activation, the cone-plate viscometer or the Couette rotational viscometer has been widely used. In a previous report, it was shown that shearing platelet-rich plasma using a Couette rotational viscometer could lead to an increase in pH by CO(2) release. However, any clear mechanism has not been provided. In this study, we examined whether shearing cell free plasma only using a cone-plate viscometer can also induce pH increase and studied the underlying mechanism of shear-induced pH increase by directly measuring total CO(2) (T(CO(2))) and CO(2) tension (P(CO(2))). When human plasma was sheared using a cone-plate viscometer, the pH of the human plasma increased time- and shear rate-dependently. Although T(CO(2)) of human plasma was not affected, P(CO(2)) was decreased by shearing, indicating that the decreased P(CO(2)) is associated with a pH increase of plasma. In addition, the pH of bicarbonate-containing suspension buffer was also shown to be increased by shearing; suggesting that the platelet studies using suspension buffers containing bicarbonate could be affected similarly. The effects of pH changes on shear stress-induced platelet activation were also investigated in the same in vitro systems. While shear stress-induced platelet aggregation was not affected by the pH changes, P-selectin expression was significantly increased in accordance with the pH increase. In conclusion, shear stress using a cone-plate viscometer induces pH increase in plasma or bicarbonate-containing suspension buffer through a P(CO(2)) decrease and the pH changes alone can contribute to platelet activation by enhancing shear stress-induced P-selectin expression.