In order to compare the effects of nitrendipine and enalapril on renal function in type-I diabetes mellitus, a randomised single-blind two-year comparative study was performed in 15 microalbuminuric patients (4 females , 11 males, age, 40 +/- 4 years, duration of diabetes, 20 +/- 4 years, Hb(A1c) 7.9 +/- 0.4%). Patients were treated over two years with either 20 mg/day nitrendipine (group 1, 9 patients) or 10 mg/day enalapril (group 2, 6 patients). Patients in both groups did not differ regarding clinical characteristics. On the average, urinary albumin excretion was decreased by 56 +/- 4% in group 1 (p<0.01 vs. before treatment) and by 52 +/- 8% in group 2 (p<0.01 vs. before treatment). Creatinine clearance rose by 24 +/- 3% in group 1 (p<0.01 vs, before treatment) but fell by 8 +/- 2% in group 2 (p<0.01 vs, group 1). Total kidney volume decreased by 24 +/- 4% (p<0.01) and by 22 +/- 10% (p<0.05), respectively. Systolic blood pressure fell by 4 +/- 1% in group 1 and by 12 +/- 1% in group 2 (p<0.01), average decrease in diastolic blood pressure was by 13 +/- 1% in group 1 and by 14 +/- 1% in group 2 (both p<0.01). In conclusion, nitrendipine long-term treatment of microalbuminuric type I diabetic patients appeared to be as effective as the treatment with enalapril in preventing or postponing the progression of diabetic nephropathy.