Hyperinsulinemia has been implicated in the pathogenesis of the blood pressure elevation in patients with noninsulin-dependent diabetes mellitus, obesity, but also essential hypertension. In these conditions an increased cardiovascular reactivity to noradrenaline (NA) and angiotensin II (AII) can be observed. Using the euglycemic clamp technique, we determined the cardiovascular reactivity to graded infusions of NA and AII in nine healthy males before (Bas), and 1 and 6 h after infusion of insulin (50 mU/kg per h) was started. On separate days control experiments were carried out to control for any circadian variation. Insulin led to a decrease of the amount of circulating NA necessary to increase the diastolic blood pressure (DBP) 20 mmHg (actual experiment [mean +/- SEM]: Bas, 23.1 +/- 5.0; 1 h, 14.8 +/- 3.0; and 6 h, 12.3 +/- 3.1; and control experiment: Bas, 20.7 +/- 5.0; 1 h, 18.6 +/- 3.5; and 6 h, 17.3 +/- 3.3 nmol/liter; Bas vs. 1 and 6 h: P less than 0.05). Although the amount of NA infused to raise DBP 20 mmHg showed a similar decline after 1 h of insulin infusion, no such change from baseline could be observed at 6 h. This appeared to be due to an increase in NA clearance with more prolonged insulin infusion. Insulin exerted no effect on the amount of AII infused to increase DBP 20 mmHg (actual experiment: Bas, 27.6 +/- 6.4; 1 h, 28.8 +/- 10.0; and 6 h, 21.2 +/- 5.3; and control experiment: Bas, 33.6 +/- 5.7; 1 h, 34.2 +/- 6.1; and 6 h, 23.4 +/- 4.7 ng/kg/min; NS). We did observe a circadian variation in AII reactivity. Whether the increase in cardiovascular responsiveness to NA after administration of insulin contributes to the elevation in blood pressure frequently observed in patients with insulin resistance remains to be proven.