It has been shown that many diseases are linked to abnormalities of the erythrocyte membrane. This study observed the changes in erythrocyte membrane fluidity during an immobilization period of 21 days. The right hindlimbs of male adult New Zealand white rabbits were immobilized for 21 days. Blood samples were collected and heparinized prior to immobilization and on days 1, 3, 7, 14, and 21 of immobilization. The membrane fluidity of erythrocytes was measured by spin-label electron spin resonance (ESR). The membrane fluidity was expressed by the value of order parameter (S). The results showed a significant increase of S on days 7 and 14 of immobilization (p <0.01). The highest value of S was found on day 7 of immobilization; thereafter, S gradually declined. Compared with the value measured before immobilization, there was no significant difference in the value of S on day 21. The results show that immobilization significantly affected erythrocyte membrane fluidity, indicating a systemic and temporal response ofthe immobilized body. This study is the first to document the time-course of changes in erythrocyte membrane fluidity in rabbits with single hindlimb immobilization during 21 days. These results may be helpful in studying the pathophysiology of immobilization.