The effectiveness of superoxide dismutase (SOD), catalase (CAT), dimethyl sulphoxide (DMSO) and allopurinol in prevention of peritoneal adhesion formation induced by complete vascular obstruction and reperfusion of an ileal segment was investigated in rats. The ischaemic period was 30 min. Group A (n = 20) were controls, group B (n = 15) received SOD 15,000 U/kg i.v. and group C (n = 17) the same dose of CAT immediately before induction of ischaemia. In group D (n = 20) DMSO 20 mg/kg was given i.v. 5 min before ischaemia, and group E (n = 20) received allopurinol orally 50 mg/kg daily for 2 days and also 2 hours before ischaemia. Ten days later adhesions had developed in 80% of group A, 40% of group B, 47% of group C and 45% of groups D and E (p less than 0.05). The severity of the adhesions was significantly less in the pretreated groups than in the controls. Oxygen-derived free radicals may be pathogenetically important for such adhesion formation. Xanthine oxidase is the principal source of oxygen radicals after a 30-min period of complete regional intestinal ischaemia.