The fluorescence polarization technique with 1,6-Diphenyl-1,3,5-hexatriene as a probe, was used to determine the lipid rotational mobility (LRM) measured by fluorescence anisotropy of isolated whole mitochondria of the rat kidney following normothermic ischemia of 30, 45, 60 and 90 minutes and upon reperfusion for 24 hours. The LRM of mitochondrial membrane lipids of the ischemic kidney decreased steadily with increasing ischemic times (0.1590 vs. 0.1705, 0.01 less than P less than 0.001 at 60 minutes). Following 24 hours reflow, there were no significant differences in the LRM of mitochondria between ischemic and control groups up to 45 minutes of ischemia, (0.1688 vs. 0.1705, 0.5 less than P less than 0.6). However, when kidney was subjected to ischemic periods longer than 60 minutes, the decreased LRM remained fixed even after reperfusion (0.1783 vs. 0.1738, 0.5 less than P less than 0.6). This suggests that 60 minutes of ischemia probably produces irreversible damage to the mitochondrial membrane whereas lesser degrees of ischemic injury is reversible upon reperfusion.