1. Streptozotocin diabetes was induced in Wistar-Kyoto rats fed a 50% protein diet. Animals were randomized to receive either the ACE inhibitor ramipril, 1 mg/L in drinking water (n = 7), or no treatment (n = 7) and were studied for 6 months. Blood glucose, body weight and glomerular filtration rate (GFR) were measured at 0, 1, 4, 8 and 16 weeks of diabetes and urinary albumin excretion was measured every 8 weeks. 2. In both groups, GFR increased significantly within 1 week of induction of diabetes (P less than 0.001) and thereafter remained stable. There was no difference in GFR between the treated and untreated groups. 3. Urinary albumin excretion increased progressively in both groups throughout the study. Ramipril treatment reduced albuminuria by approximately 50% at weeks 16 and 24 (P less than 0.01). 4. The amelioration of diabetic albuminuria by ACE inhibition, in the setting of high dietary protein intake, may have important implications for the treatment of human diabetic nephropathy.