Rheological alterations produced during the storage of human erythrocytes were studied by means of in vitro viscoelastic parameters. Weekly aliquots of packed red blood cells (PRBC) and whole blood (WB) stored in CPD-adenine were studied for 35 days. Samples were analyzed in Erythrodeformeter both in stationary and oscillating regime. We found that erythrocyte deformability index and all complex viscoelastic parameters are modified by storage. Plasma viscosity and blood viscosity were measured in a cone/plate Viscometer. Blood viscosity at high shear rates shows variations related to conservation time, which are in correlation with the variations observed in the erythrocyte deformability index. Blood viscosity at low shear rate decreases during storage. Rheological tests would demonstrate some alterations in the structural properties of erythrocyte stored as PRBC. These alterations are less than those found in WB. We conclude that plasmatic proteins play a predominant role in the alteration of rheological behavior of erythrocytes during their storage. Consequently, it is important to evaluate not only cellular components but also plasma.