The status of lipid peroxidation, glutathione, glutathione peroxidase, glutathione reductase, glutathione-S-transferase, superoxide dismutase, catalase, ascorbic acid, and a-tocopherol was studied in the urinary bladder of guinea pigs exposed to the carcinogenic fern Onychium contiguum. There was significant increase in the preformed lipid peroxides in the urinary bladders from fern exposed animals. The amount of lipid peroxides produced on incubation of urinary bladder homogenates with or without catalyst was significantly higher in the fern exposed animals. The concentrations of glutathione and alpha-tocopherol and the activities of glutathione reductase and catalase were elevated in the urinary bladders of the animals exposed to the fern. No effect was observed on the concentration of ascorbic acid and the activities of glutathione peroxidase, glutathione-S-transferase, and superoxide dismutase. It is summarized that the fern toxins increased oxidative stress in the urinary bladder and antioxidant status was altered. However, the altered antioxidant status did not provide protection from the toxin induced injury. Histopathology of the urinary bladder in the fern exposed animals revealed oedema, haemorrhages, and congestion. This is the first study to show increase in lipid peroxidation along with altered antioxidant status in the urinary bladder of fern exposed animals. (C) 2003 Elsevier Science (USA). All rights reserved.