In diabetes mellitus (DM), the urine flow rate is increased, and the fluid turnover in the body is accelerated because of the glucose-induced osmotic diuresis. On the other hand, plasma vasopressin (VP) is elevated in both type 1 and type 2 DM. This elevation seems to be due to a resetting of the osmostat. A high VP level is beneficial in the short term because it limits to some extent the amount of water required for the excretion of a markedly enhanced load of osmoles (mainly glucose). However, in the long run, it may have adverse effects by favoring the developement of diabetic nephropathy. VP has been shown in normal rats to induce kidney hypertrophy, glomerular hyperfiltration, and an increase in urinary albumin excretion (features also occurring in association in the period preceding diabetic nephropathy). Moreover, VP has been shown to participate in the progression of renal failure in rats with five-sixths reduction in renal mass. In recent studies, we have shown (1) that creatinine clearance, albuminuria and renal mass increased much less during experimental DM in Brattleboro rats unable to secrete VP than in their VP-replete Long-Evans controls, and (2) that albuminuria was prevented during experimental DM in Wistar rats when a VP nonpeptidic, highly selective V2 receptor antagonist was administered chronically for 9 weeks. Taken together, these results strongly suggest that VP plays a crucial role in the onset and aggravation of the renal complications of DM. The mechanisms by which VP exerts these adverse V2-dependent effects are not yet elucidated. They are most likely indirect and may involve several intermediate steps comprising VP-induced changes in the composition of the tubular fluid in the loop of Henle (due to solute recycling in the renal medulla associated with improved concentrating activity of the kidney), inhibition of the tubuloglomerular feedback control of glomerular function, and alterations in glomerular hemodynamics by the intrarenal renin-angiotensin system.