To investigate the potential effects of variation of HO-1 activity on hemorheology, this study compared the hemorheological properties between transgenic HO-1G143H mutant mice and wild-type (WT) control mice. Fresh blood samples were obtained from mice via the ocular venous sinus. The whole blood viscosity was measured using a cone-plate viscometer. Erythrocyte deformability and aggregation was measured using ektacytometry. The elongation index was significantly reduced in the HO-1G143H mutant mice compared to the WT mice at the shear rates of 600, 800, and 1,000 s(-1). The integrated elongation index was decreased in the HO-1G143H mutant mice compared to the WT mice. There was no statistically significant difference between the HO-1G143H mutant mice and the WT mice in terms of whole blood viscosity, aggregation index, amplitude of aggregation, and aggregation half time. The present study demonstrated that a reduction in HO-1 activity results in an impaired erythrocyte deformability. Although the mechanism underlying this effect remains unclear, our study brings to light the participation of HO-1 in the variations of hemorheology.