We investigated the influence of partial oxidative stress on permeability and fluidity of nucleated fish red blood cells for simulating nucleated somatic cells. Peroxide value indicating lipid hydroperoxide level in nucleated red blood cells of common carp (Cyprinus carpio) increased with increasing body size. We detected that oxidation of nucleated red blood cells led to the degraded PUFA compositions and accelerated the permeability of calcein and ATP in the nucleated red blood cells restrictedly oxidized with 1 mM AAPH treatment for 30 min at 21 degrees C in the dark. Using fluorescence probes, PC3P, we found that oxidative stress reduced the membrane fluidity of nucleated red blood cells. It was also observed that AAPH had no significant influence on the osmotic fragility and electrophoretic profiles of red blood cell proteins. These results suggest that partial oxidative-stress, even if failure to fragment the membrane, may affect membrane permeability of fish nucleated red blood cells for an important energy molecule, ATP.