An association between hyperhomocysteinemia and premature atherosclerosis in patients with non-insulin-dependent diabetes mellitus (NIDDM) has recently been described. Little is known about the role of insulin in homocysteine [H(e)] metabolism. We measured plasma H(e) concentrations in the fasting state and during a hyperinsulinemic-euglycemic clamp in normal subjects and patients with NIDDM. Plasma H(e) decreased significantly from 7.2 +/- 2.6 to 6.0 +/- 2.7 mmol/L (P < .01) in normal subjects, but did not change in patients with NIDDM (6.0 +/- 2.7 to 5.9 +/- 2.5 mmol/L, respectively). These data suggest that plasma H(e) concentrations are regulated by acute hyperinsulinemia in normal subjects, but not in insulin-resistant NIDDM subjects. These abnormalities may have implications for the pathogenesis of premature vascular disease associated with NIDDM.