We measured plasma- and extracellular fluid volume (125I-albumin, 51Cr-EDTA), plasma concentrations of renin, angiotensin I and II, aldosterone and atrial natriuretic peptide by radio-immunoassays in insulin-dependent diabetic (IDDM) patients with (n=28) and without (n=11) nephropathy and in 14 normal control subjects matched for sex and age. Glomerular filtration rate (GFR) (ml/min/1.73 m2, single intravenous bolus 51Cr-EDTA technique) was within normal range in all nephropathic patients; 107 (range 78-134). Mean arterial blood pressure (mmHg) was elevated 102 +/- 13 (+/- S.D.) compared to the diabetic and normal control group, 92 +/- 8 and 87 +/- 5, respectively (p less than 0.01). Plasma volume was identical in all three groups while extracellular volume (1/1.73 m2) was expanded in nephropathic patients, 14.5 +/- 1.5 vs 13.1 +/- 0.9 and 12.4 +/- 1.3 in the diabetic and non-diabetic control groups, respectively (p less than 0.05). A significant correlation between extracellular fluid volume and mean arterial blood pressure was found (n=53, r=0.49, p less than 0.001). Active renin was significantly increased in patients with diabetic nephropathy compared with the normal control subjects, while all the remaining hormones were about the same in the three groups. Our study suggests that fluid retention plays a dominant role in the initiation and maintenance of arterial blood pressure elevation early in the course of diabetic nephropathy.