Red blood cell (RBC) deformability in rats of various ages was assessed by filtration (3 micron Nucleopore membranes). Neonatal rat RBC (1 d old) had lower filterability, both in terms of RBC incremental volume (9.97 +/- 1.85 versus 0.33 +/- 0.28 nL at 180 d of age, mean +/- SD, p less than 0.001) and the number of filter clogging particles (25.7 +/- 3.1 versus 18.9 +/- 3.4 RBC x 10(3)/s, p less than 0.001). The lower filterability correlated with a larger RBC volume (169 +/- 12.6 versus 69 +/- 3.2 microns 3, p less than 0.001) and with a wider minimum cylindrical diameter (3.87 +/- 0.15 versus 2.84 +/- 0.05 microns, p less than 0.001). Almost all of the neonatal RBC had a minimum cylindrical diameter exceeding the 3 microns nominal pore size of the filters. The calculated resistance to initial folding was also significantly greater, as indicated by a static bending analysis of initial deformation. However, when the larger size of neonatal RBC was taken into consideration, and thus their greater projected area on which forces are applied, they appear to be at least as deformable as the adult type RBC. This finding may explain the contradiction between RBC filtration experiments and other approaches based on RBC deformations in shear flow, which have been unable to detect a hampered flexibility of neonatal RBC. In view of the more pronounced differences between neonatal and adult RBC in rats than in human subjects, the rat is an interesting model for studying this physiologic phenomenon in newborns.