The effect of acute oral selenium toxicosis on the rate of lipid peroxidation, on the amount of reduced glutathione as well as on glutathione-peroxidase activity of the blood (plasma and red blood cells) and liver was studied in broiler chicken. Cockerels (28-day-old) were treated with selenium (4.85 mg/kg b.w.) administered intraoesophageally in the form of sodium selenite. Samples were at the onset of clinical signs (3 h after treatment) and 2 and 4 h thereafter. The malondialdehyde content of the blood plasma rose significantly (P < 0.05) at the onset of clinical symptoms but decreased later. Malondialdehyde content of the liver was higher than the control value at the first sampling (P < 0.01) and steadily increased later. Reduced glutathione content did not change significantly in the blood plasma and liver. Glutathione peroxidase activity of the RBC was significantly elevated (P < 0.01) only at the first sampling (3 h after treatment) and decreased to the control level thereafter. Acute oral selenium toxicosis increases the rate of lipid peroxidation in a short period of time (7 h) without exerting a significant effect on the glutathione system.