1. The nephrotic syndrome was induced in inbred female Wistar rats by the intravenous injection of puromycin aminonucleoside (PA) (5 mg/100 g body weight). 2. One group (n = 12) received superoxide dismutase (SOD) (15 mg/kg body weight), a second group (n = 12) received polyethylene glycol coupled catalase (PEG-catalase) (5000 i.u./kg body weight) and the third (n = 9) saline (150 mmol/l NaCl) via the intraperitoneal route, in addition to the PA. 3. SOD and PEG-catalase reduced the 24 h urine protein on days 8 and 15 compared with unmodified puromycin treated animals and this difference was significant on day 15 for SOD (P less than 0.05) and for PEG-catalase (P less than 0.01). Glomerular filtration rate, as measured by the creatinine clearance, was lower in the PEG-catalase group but did not differ significantly from the saline treated group. 4. These data suggest that superoxide anion and hydrogen peroxide, or their reaction products, are involved in the glomerular injury of puromycin nephropathy.