Plasma epinephrine may be increased in some patients with essential hypertension, and prolonged infusion of this catecholamine has been claimed to raise blood pressure. The objectives of our experiments were to determine whether continuous intravenous infusion of epinephrine raised blood pressure in a rat preparation known to respond in this way to angiotensin II in low dose and, if so, the plasma concentration of epinephrine required to raise pressure in comparison to the physiological range of plasma catecholamine concentration in the rat. Intravenous infusion of epinephrine at a rate of 400 ng X kg-1. min-1 or less for 30 min into Wistar rats did not increase mean arterial pressure (MAP); pressure did rise when the rate was increased to 800 ng X kg-1. min-1. However, when rats were given epinephrine at 400 ng X kg-1. min-1 for 4 days with continuous blood pressure recording, average MAP showed a progressive rise on successive days of infusion, reaching a maximum increase of 12 mmHg on the 4th and final day of infusion (P less than 0.02). Blood pressure did not change significantly during epinephrine infusion at 10 and 70 ng X kg-1. min-1. Plasma epinephrine was raised more than 13 times basal at the highest rate of infusion. In comparison, blood pressure and catecholamine concentrations increased only slightly although significantly during a period of restraint. We confirm the existence of a slowly developing pressor effect of epinephrine, but it is small and requires a large sustained increase of plasma epinephrine for its development.