Clinical studies indicate a nephro-protective effect in conjunction with the use of ACE inhibitors. This study's aim was to determine whether ACE inhibitors influence the metabolism of glomerular cells in addition to their known hemodynamic effects. Streptozotocin diabetic rats were treated with lisinopril (DLis 1.5 mg/l water), or hydralazine (Dhyd, 50 mg/l water) over 4 weeks. Untreated diabetic rats (DC) and non-diabetic rats (C) served as controls. After four weeks of treatment, urinary excretion of albumin, blood pressure and metabolic control (Glyc-Hb) were measured. After treatment glomeruli were isolated and homogenized, and beta-NAG and total proteolytic activity against azocasein were measured. Glycated hemoglobin levels were similar in all diabetic groups (DC, 12%, Dhyd, 10%; DLis 11%). Blood pressure of DLis rats (79 +/- 3 mmHg) and DHyd rats (46 +/- 2 mmHg) was lower than that of DC rats (111 +/- 3 mmHg). Urinary albumin excretion of diabetic groups was lowest in DLis. Diabetic rats showed a decrease in glomerular beta-NAG (10 vs. 60.5 U/g protein) and total proteolytic activity against azocasein (148 vs. 170 U/mg protein hour) compared to non-diabetic rats. Lisinopril increased beta-NAG (30 vs. 14 U/g protein) and total proteolytic activity (160.5 vs. 141.5 U/mg protein hour) compared with hydralazine. Our study confirms that the nephro-protective effect of ACE inhibitors is partially due to modulatory effects on the metabolism of basement membrane proteins.