Previous reports have demonstrated that red blood cells (RBC) have an active nitric oxide (NO) synthesizing mechanism which has properties similar to endothelial nitric oxide synthase (eNOS). This red cell NOS activity contributes to the NO export from RBC. The present study explored the influence of shear stress applied to RBC on NO concentrations of cell suspensions. RBC were exposed to shear stress by filtration through 5 microm diameter pores under 10 cm H2O pressure, generating a wall shear stress of approximately 110Pa. NO concentration in the RBC suspensions were measured using electrochemical NO probes before and after filtration through the micropores. NO concentration was found to be significantly increased after a single passage of RBC suspensions through the micropores. The increment in NO concentration depended on the presence of calcium, being 21.8+/-4.4 nM with 1 mM calcium and 13.7+/-2.7 nM without added calcium. Including the calcium chelator EDTA completely abolished this increase. The increment of NO was also affected by the level of oxygenation, being more pronounced under hypoxic conditions. These results confirm that RBC NO generating mechanisms can be stimulated by exposing red cells to shear stress and that calcium plays a role in this stimulation.