Background. Peritonitis is a major complication in continuous ambulatory peritoneal dialysis (CAPD) patients, and the mechanisms involved in the pathology are important if the success rate of treatment strategies is to increase. Material and methods. A total of 50 CAPD patients (25 with 25 episodes of peritonitis and 25 with no clinical or laboratory signs of infection) were included in the study. Malondialdehyde (MDA) and nitric oxide (NO) metabolites in serum and dialysate effluents were determined. Results. The dialysate/serum (D/S) ratio of the NO metabolites and serum NO metabolite concentrations were significantly higher in the peritonitis group. Serum and dialysate MDA concentrations were also significantly higher in the peritonitis group. The D/S ratio of MDA was significantly higher in the control group. Conclusions. Local peritoneal NO production and oxidative stress seem to increase in CAPD patients during the peritonitis attack.