The effect of captopril on proteinuria was evaluated in twenty patients with various glomerular diseases excreting heavy proteinuria (> 3.0 g/day). Captopril in a daily dose of 37.5 mg was administered orally three times a day to all patients and they were followed for eight weeks. Twenty-four hour urinary excretion of protein, creatinine, sodium, selective protein index (SPI), and blood chemistry including serum electrolytes were measured every two weeks. Twenty-four hour urinary protein excretion per gram creatinine started to fall within two weeks of captopril administration and became nearly stable after four weeks of therapy (p < 0.05). Mean 24-hour urinary protein excretion decreased significantly from a pretreatment value of 9.0 +/- 6.0 gm/gm of cr. to 4.4 +/- 3.5 gm/gm of cr. after eight weeks of captopril treatment. The serum albumin level increased progressively at six and eight weeks after the captopril treatment period and was significantly higher than the pretreatment value (p < 0.05). The decrease in proteinuria did not coincide with a fall in blood pressure or any changes in creatinine clearance. We conclude that captopril does have a significant antiproteinuric effect in patients excreting heavy proteinuria with various glomerular diseases. However, the long term therapeutic efficacy and any renal protective effect of this drug remain to be proven.