Eighteen children with steroid-sensitive nephrotic syndrome (SSNS) were studied. The control group comprised 20 healthy children. The following indirect parameters of reactive oxygen species activity were determined in nephrotic patients during four stages of the disease (full relapse before prednisone administration, disappearance of proteinuria, prednisone cessation, unmaintained remission): plasma malondialdehyde (MDA) levels, copper/zinc superoxide dismutase (CuZn SOD) activity and glutathione peroxidase (GPX) activity in erythrocytes, reduced glutathione (GSH) and vitamin C levels in whole blood, and vitamin E level in serum. Increased MDA levels, reduced vitamin C levels, and enhanced CuZn SOD activity were found in relapse. GSH concentration was high during all four stages. Vitamin E level was also increased, parallel to the pattern of serum lipids. GPX activity remained low during the proteinuria stage and in remission. We conclude that the majority of abnormal findings can be attributed to the hyperlipidemia of NS. Low GPX activity may be a factor limiting the antioxidant capacity in NS. The present study is inconclusive regarding the role of free radicals in the proteinuria of NS.