The aim of this study was to examine the effects of vitamin C on malonaldehyde (MDA) and glutathione (GSH) concentrations, and superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) activity in chickens stressed by adrenocorticotropin (ACTH). Sixty Leghorn chickens (20 weeks old) were randomly allotted to a control and a vitamin C treated group. An isotonic sodium chloride solution was administrated intramuscularly for a period of five days to the control group at a dose of 2.5 mL per chicken per day, and the treatment group received a vitamin C solution (containing 250 mg vitamin C/2.5 mL) intramuscularly for five days. On the fifth day of the experiment the chickens in both groups received 50 IU ACTH intramuscularly. Three hours after ACTH application blood samples were collected to determine the concentrations of MDA and GSH, and activities of SOD and GSH-Px in the blood. The concentration of MDA in the control group increased compared to that in the vitamin C treated group. After the ACTH application the activity of SOD increased in both groups, thought significantly so in the vitamin C treated group. The GSH-Px activity after ACTH application was not significantly different between the groups. It was concluded that intramuscularly administrated vitamin C facilitated an adaptation against stress and decreased negative effects of stress in chickens.