A large body of correlational evidence relating plasma insulin levels and arterial pressure in obese hypertensives suggests that hyperinsulinemia may play a causal role in the development of hypertension in these subjects. However, experimental evidence supporting the ability of increased plasma insulin per se to increase blood pressure is lacking. The goal of this study was to determine the effect of hyperinsulinemia on mean arterial pressure and renal electrolyte excretion in eight conscious rats. Arterial pressure was determined by sampling the signal from an abdominal aortic catheter once per minute, 19 h/day by computer. A 5-day intravenous insulin and glucose infusion that increased plasma insulin concentration 43% significantly increased mean arterial pressure from 93 +/- 1 mmHg to an average of 101 +/- 2 mmHg for the 5-day experimental period. Heart rate increased from 369 +/- 8 to 406 +/- 3 beats/min. Urinary sodium excretion transiently decreased on day 1 of insulin, but no significant sodium retention was measured after 5 days of insulin infusion, suggesting that the blood pressure increase was not volume mediated. There were no changes in any of these variables in eight vehicle-infused rats. These results suggest that hyperinsulinemia can increase mean arterial pressure in conscious rats, but the underlying mechanism remains to be elucidated.