1. Uncomplicated insulin-dependent diabetes mellitus is associated with generalized vasodilatation. This vasodilatation is believed to contribute to the development of microvascular complications. The endothelium plays an important role in the regulation of vascular tone. 2. To investigate the role of endothelial mediators, we measured plasma endothelin levels and studied the vascular effects of intravenous L-arginine (the precursor of NO) in 10 male type 1 diabetic patients and 10 non-diabetic subjects. 3. The baseline plasma endothelin level was significantly lower in the diabetic patients [mean 1.7 (SD 0.5) versus 2.1 (0.4) pmol/l; P < 0.05] than in the control subjects. 4. During L-arginine infusion, plasma cyclic GMP (the second messenger for NO) increased in the control subjects [from 5.1 (2.9) to 6.9 (2.9) nmol/l; P < 0.05 versus saline] and in the diabetic patients [from 4.6 (1.8) to 5.7 (2.2) nmol/l; P = 0.09]. L-Citrulline (a by-product of NO synthesis from L-arginine) increased in both groups. The responses to L-arginine were not significantly different between the control subjects and the diabetic patients. The plasma atrial natriuretic peptide level did not change in either group during infusion of L-arginine or of an equal volume of isotonic saline. 5. Blood pressure decreased slightly during L-arginine administration in both groups. In control subjects, the extracellular fluid volume in the lower leg increased during L-arginine infusion as compared with saline; in the diabetic patients both L-arginine and saline increased the extracellular fluid volume.(ABSTRACT TRUNCATED AT 250 WORDS)