Impaired renal sodium excretion and increased plasma atrial natriuretic peptide (ANP) levels have been reported in patients with hypertension associated with insulin resistance and hyperinsulinemia. To clarify the interrelationship between hyperinsulinemia and plasma natriuretic peptides, we investigated the effects of physiological and non-physiological hyperinsulinemia on the plasma ANP and brain natriuretic peptide (BNP) levels. Plasma immunoreactive insulin (IRI), ANP and BNP levels were determined by a euglycemic-hyperinsulinemic glucose clamp in 20 patients with non-insulin-dependent diabetes mellitus, by a glucose challenge test in 22 normal subjects and by an insulin challenge test in six normal subjects. Both in the glucose clamp and the glucose challenge test, plasma ANP showed a significant increase in association with increased plasma IRI and plasma volume. However, there was no significant correlation between the changes in plasma ANP levels and plasma IRI levels in view of the peak values and the area under the curve of their responses. In addition, the plasma ANP did not show any significant change despite the marked elevation of plasma IRI in the insulin challenge test. There was no significant change in plasma BNP under any of the hyperinsulinemic conditions. These findings provide in vivo evidence for the lack of a direct effect of acute hyperinsulinemia on natriuretic peptides, although the chronic effects of hyperinsulinemia remain to be elucidated.