In 30 diabetics (15 with type I and 15 with type II) and in 15 healthy controls the effect was studied of two-hour immersion in water on the concentration of atrial natriuretic peptide, sodium and potassium in serum, diuresis and natriuresis, mean arterial pressure and percent change of circulating plasma volume. Diabetics with either type of diabetes differed from the controls by a significantly lower concentration of the atrial natriuretic peptide under basal conditions, and lower rise of the peptide during immersion in water. Type I diabetics differed from type II diabetics and controls in that their natriuresis was lower under basal conditions and during immersion. On the basis of these results the authors conclude that in diabetics the reaction of atrial natriuretic peptide secretion to central hypervolaemia is preserved but weaker, and the role of this hormone in the pathogenesis of increased diuresis and natriuresis induced by water immersion is smaller in these patients than in controls.