1. Nephrotoxic serum nephritis was induced immunologically in uninephrectomized Sprague-Dawley rats (n = 24). One-half were assigned randomly to treatment with enalapril (5 mg/kg per 24 h). 2. Untreated rats (n = 12) developed a progressive fall in creatinine clearance, a progressive rise in systolic blood pressure and marked proteinuria over a 6 week period. The mortality rate was 42% at 5 weeks and 66% at 6 weeks. 3. Enalapril-treated rats (n = 12) had no significant reduction in systolic blood pressure, and a similar reduction in creatinine clearance to that in untreated rats. Mortality was 50% at 5 weeks, and 92% at 6 weeks. 4. Proteinuria but not the blood pressure rise was significantly reduced by enalapril treatment. 5. Converting enzyme inhibitors may have a specific role in the treatment of nephritic diseases.