The present study was designed to investigate the effect of captopril, a sulfhydryl (-SH) containing ACE inhibitor and lisinopril, a non-SH containing ACE inhibitor, on ischaemia-reperfusion-induced renal injury in rats and to study the involvement of the free radical scavenging property of captopril in its renoprotective effect. Bilateral renal artery occlusion was induced for 30 min followed by reperfusion for 24 h. Blood samples were taken from retro orbital sinus before surgery and at 24 h after reperfusion for blood urea and blood creatinine estimation. After completion of 24 h of renal reperfusion the carotid artery was cannulated and the mean arterial blood pressure (MABP) was recorded. The left kidney was used for histological examination. The right kidney was utilised for estimation of mitochondrial thiobarbituric acid reactive substances (TBARS). Renal ischaemia, followed by reperfusion, significantly increased blood urea nitrogen (BUN) and blood creatinine. However, creatinine clearance decreased markedly. Captopril administered before renal artery occlusion or immediately after reperfusion and lisinopril pre-treatment significantly attenuated the increase in BUN and blood creatinine. Creatinine clearance was markedly better in captopril-treated animals as compared to lisinopril-treated rats. Captopril significantly decreased the degree of tubular necrosis, haemorrhagic streaks and urinary casts. Lisinopril treatment decreased tubular necrosis and urinary casts but no marked effect on haemorrhagic streaks was noted. Administration of captopril before ischaemia or just after reperfusion significantly reduced the elevated concentration of mitochondrial TBARS but no such decrease was noted in lisinopril-treated rats. Based on these results it may be concluded that captopril and lisinopril markedly protected against ischaemia-reperfusion-induced renal injury and any additional renoprotective effect of captopril may be ascribed to its free radical scavenging properties.