The effect of schisandrin B (Sin B) on oxygen free radicals--induced lipoperoxidative damage to plasma membrane of rat hepatocytes was investigated. When the plasma membrane of rat hepatocytes was incubated with iron/cysteine or Vit C/NADPH, the production of malondialdehyde (MDA) and consumption of NADPH increased, while the membrane fluidity reduced. Addition of Sin B (3-25 micrograms.ml-1) to the incubation mixture inhibited all these alterations of the plasma membrane induced by iron/cysteine and Vit C/NADPH. The results indicated that Sin B could maintain membrane stability of rat hepatocytes under oxidative stress.