Red cell deformability of 30 nonpregnant volunteers and 20 normal pregnant women in the 3rd trimester was assessed by determination of filterability of red cells suspended either in autologous plasma or in buffer by St. George’s nitrometer method. Total red cell deformability was decreased in normal pregnancy [transit time (Tc) = 7.79 ± 0.86 vs. 6.99 ± 0.65, p < 0.01] as compared to nonpregnant women when erythrocytes were studied in buffer suspensions, while the number of profoundly rigid red cells was lower in pregnant patients [clogging particles (CP) = 0.864 ± 0.225 vs. 1.103 ± 0.246, p < 0.01]. Total red cell deformability was no longer reduced in pregnant women when erythrocytes were suspended in autologous plasma (Tc = 7.25 ± 0.66 vs. 7.13 ± 0.69, p = NS) while the number of rigid erythrocytes was still lower (CP = 0.802 ± 0.157 vs. 1.055 ± 0.210, p < 0.01). Our data suggest that by the end of normal pregnancy, red cell deformability decreases with a consequent accelerated turnover and a fast elimination of very poorly deformable erythrocytes; plasma alterations in late pregnancy partly counteract the intrinsic loss of deformability of red blood cells.