Abstract: The objective of this study was to evaluate mechanisms of microcystin (from King Talal Reservoir, Jordan) toxicity on the hepatocytes of mice. Five to seven weeks old male albino Balb/C mice (about 30 g wt.) were injected i.p. with normal saline (as control) or Microcystis aeruginosa aqueous extract (30 mg L-1) at 24 h intervals for 48 h. After the exposure period of toxin-treated and control mice, Na+, K+-ATPase and glutathione-S-transferase (GST) activities were measured in hepatocytes extracts. Total oxyradical scavenging capacity (TOSC) and lipid peroxides (LPO) content were also determined. Na+, K+-ATPase and GST in the hepatocytes of mice was significantly lower in toxin treated mice than in control ones. Mice exposed to microcystin also showed a significant increase in the TOSC value against peroxyl radicals. Lipid peroxides level did not change in the hepatocytes of mice after exposure to the toxin. The increased levels of TOSC suggest on the occurrence of mice response against oxidative stress induced by toxin injection, which prevents lipid peroxidation.