The micropipette test was used to study the effects of age on the elasticity of red blood cell (RBC) membrane in spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar-Kyoto rats (WKY), ranging from 3 to 23 weeks of age. The development of hypertension in the SHR started at 3 weeks and was fully established at 7 to 8 weeks. In the developmental phase of hypertension (3-5 weeks), the SHR showed a significant increase in RBC membrane elastic modulus (i.e., a decrease in RBC membrane deformability) when compared with the age-matched normotensive control rats (WKY). After the establishment of hypertension (7-8 weeks), however, the deformability of the RBC membrane of SHR improved and became comparable to that of the WKY. These results indicate that abnormal erythrocyte membrane elasticity is an early event in SHR and that adaptive recovery occurs when hypertension is fully developed.