Adrenaline was infused intravenously into conscious dogs to test whether chronically elevated plasma levels can produce arterial hypertension. Adrenaline infused at 12.5 micrograms/kg/hr from 4 days produced no change in mean arterial pressure (1.1 +/- 2.7 mmHg) despite raising plasma adrenaline concentrations from 49 +/- 20 pg/ml to 1420 +/- 279 pg/ml. Myocardial tissue levels of adrenaline averaged 435 ng/g, compared to 20 ng/g in vehicle infused dogs. Longer infusions of adrenaline (11 days) at doses from 1.25 micrograms/kg/hr to 12.5 micrograms/kg/hr were also without significant effect on arterial blood pressure. In contrast, infusion of noradrenaline at 6.25 micrograms/kg/hr for 11 days produced sustained elevation of mean arterial pressure (11.7 +/- 5.1 mmHg). Although adrenaline infusion alone did not alter arterial pressure, a small rise (5.6 +/- 1.6 mmHg) was measured when the dogs were also given cortisone (50 mg twice daily). These results are therefore not in accord with the hypothesis that increased plasma levels of adrenaline may cause hypertension by activation of pre-junctional beta-adrenoceptors. However simultaneous administration of adrenaline and cortisone did elevate blood pressure, indicating that increases in both adrenal cortical and medullary hormones may be required to produce hypertension.