We evaluated the efficacy of an ACE inhibitor captopril (CAP) for the reduction of proteinuria in glomerular diseases, and tried to find the conditions in which urinary protein excretion was significantly decreased by this drug. Renin provocation test by CAP (C-test) was performed, and the result was compared to the effect on proteinuria. In 33 patients with proteinuria, ranging from 1.1 to 14.1 g/day, CAP was administered. Urinary protein excretion was reduced from 3.6 +/- 0.6 to 2.8 +/- 0.4 g/day (mean +/- SEM, p less than 0.01) after 2 weeks. The decrease in urinary protein was significant when renal function was moderately impaired (30 less than or equal to Ccr less than 60 ml/min) or patients were on a salt diet less than 7 g of NaCl daily. Reduction of urinary protein excretion by 2-week treatment of CAP was correlated with the result of C-test (r = 0.874, p less than 0.025). The long-term follow up for more than 6 months also suggested that CAP delayed the deterioration of renal function. Thus, CAP was proved effective in treating proteinuria, and C-test might give us an information of its proteinuria-suppressing effect in an individual case. But its efficacy was observed only in patients with moderately-reduced renal function or on low-salt diet. Therefore, we should select the cases carefully to expect the effect of CAP for the reduction of proteinuria.