Three nonsteroidal anti-inflammatory drugs were tested: two inhibitors of COX-1 and COX-2 (acetylosalicylic acid and diclofenac), one inhibitor of COX-2 (nimesulid) and superoxide dismutase, catalase and glutatione peroxidase activity was estimated. The investigations were carried out in rats fed by gastric tube for three weeks with acetylsalicylic acid in doses of 2 and 10 mg/kg body weight, diclofenac in doses 1 and 5 mg/kg body weight and nimesulid in doses 2.5 and 12.5 mg/kg body weight. The results were compared with a control group of rats which obtained water into the stomach. No statistically characteristic changes of superoxide dismutase activity were observed. Catalase activity was statistically decreased after both doses of nimesulid and acetylsalicylic acid at the dose of 10 mg/kg body weight. Both doses of diclofenac increased catalase activity. Glutatione peroxidase activity was statistically decreased after both doses of nimesulid and in the dose 10 mg/kg body weight of acetylsalicylic acid and in the dose 5 mg/kg body weight of diclofenac.