The effect of the converting enzyme inhibitor (CEI) (captopril, 50 mg/kg/day) on proteinuria (UProt), urinary aldosterone (UAldoV), plasma renin activity (PRA), plasma renin concentration (PRC), plasma angiotensinogen concentration (PAC), urinary sodium (UNaV), serum total protein, and body weight was studied for 21 days in an experimental nephrotic syndrome (NS) model induced in rats by a single injection (15 mg/100g) of puromycin aminonucleoside (PA). The effect of captopril on control rats without NS was also characterized. In control rats, captopril increased PRC and PRA, and decreased PAC; it had no effect on UNaV, UAldoV, UProt, total serum protein and body weight. In rats with NS, captopril had no effect on sodium retention, hypoproteinemia, and UProt; it abolished the increased UaldoV and favored weight loss. Captopril also rose PRA and PRC, and decreased PAC in PA-nephrotic rats; these changes were similar to those produced by captopril in control rats. The mortality rate was higher in nephrotic rats treated with captopril (37%) than in untreated nephrotic rats (13%). It is concluded that captopril has no beneficial effects on the course on NS induced by PA during the first 21 days, and supports the contention that sodium retention is not related to the renin-angiotensin-aldosterone system activity in these rats.