The mitochondrial membrane fluidity in the rat liver and kidneys was measured by electron spin resonance spectrometry using spin labels 5- and 16-doxyl stearic acid (5-, 16-DS) after administration of an ethanol solution (7%) for 3 weeks. The order parameter, calculated from the 5-DS spectra, which is utilized for assessing the fluidity of the lipid bilayer near the surface of the membrane, increased significantly in the liver of the ethanol-administered group compared with the control group. No significant difference in the order parameter was observed in the kidney. On the other hand, in the motion parameter from the 16-DS spectra, which is utilized for the core of the lipid bilayer, no significant change was observed in either the liver or the kidneys. These findings indicated that the mitochondrial membrane fluidity decreased, by chronic ethanol administration, in the lipid bilayer near the surface in the rat liver, but no fluidity change was observed in the rat kidney.