The effect of cyclosporin (CS) on intrinsic glomerular level of antioxidants in puromycin aminonucleoside (PAN) nephrosis was examined. A single intravenous dose of PAN (50 mg/kg body weight) given to Sprague-Dawley rats resulted in marked proteinuria. Ten days after PAN injection, the rats were treated with daily intraperitoneal injection of CS (10 mg/kg body weight/day) for 10 days. PAN-treated rats without CS treatment (PAN rats) had significantly lower activities of glomerular superoxide dismutase (SOD) and catalase (CAT) than normal rats (p < 0.05, respectively). When compared with PAN rats, CS-treated PAN rats had significantly less proteinuria and higher activities of glomerular SOD and CAT (p < 0.01 and p < 0.05, respectively). Significant elevation of glomerular malondialdehyde (MDA) level characteristic of PAN rats was absent in CS-treated PAN rats. Moreover, segmental sclerosis with capsular adhesion, hyalinosis, epithelial cell foot process fusion and microvillous transformation seen in PAN rats were apparently attenuated in CS-treated PAN rats. When compared with normal rats, rats receiving CS only had a significantly higher CAT activity and MDA level (p < 0.01 and p < 0.05, respectively). Assessment of glomerular reduced glutathione revealed no significant differences among PAN rats, CS-treated PAN rats, normal rats, and rats receiving only CS. These data indicate that glomerular antioxidant enzyme activities are modulated by CS.