The aim of the study was to clarify whether antihypertensive treatment with a selective beta blocker would have an effect on the progression rate of kidney disease in patients with incipient diabetic nephropathy. Six male patients with juvenile-onset diabetes with incipient nephropathy (urinary albumin excretion above 15 micrograms/min and total protein excretion below 0.5 g/24 hr) were treated with metoprolol (200 mg daily). At the start of the antihypertensive treatment the mean age was 32 years +/- 4.2 (SD). The patients were followed a mean 5.4 years +/- 3.1 (SD) with repeated measurements of urinary albumin excretion before and during 2.6 years +/- 1.0 (SD) of treatment. The blood pressure was depressed by the treatment (systolic blood pressure from 135 mm Hg +/- 8.6 to 124 mm Hg +/- 6.2, NS; mean blood pressure from 107 mm Hg +/- 7.6 to 97 mm Hg +/- 3.4, 2p less than 0.05; diastolic blood pressure from 93 mm Hg +/- 9.1 to 84 mm Hg +/- 3.6, 2p less than 0.05. Albumin excretion decreased (131.0 micrograms/min X/divided by 2.9 [geometric mean X/divided by tolerance factor] to 56.1 micrograms/min X/divided by 3.7, 2p less than 0.02). The mean yearly increase in urinary albumin excretion before treatment was 18 +/- 17 (mean +/- SD). Albumin excretion decreased during treatment: 17% +/- 15 per year (mean +/- SD, 2p less than 0.02). No changes were seen in glomerular filtration rate or renal plasma flow (149 ml/min +/- 5.8 vs 144 ml/min +/- 11.1, and 516 ml/min +/- 31.0 vs 541 ml/min +/- 68.5 respectively [n = 5]).(ABSTRACT TRUNCATED AT 250 WORDS)