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Conscious male beagle dogs were given constant intravenous infusions of noradrenaline for 14 days, four receiving 125 ng/kg/min and four 250 ng/kg/min. Before, during and after these infusions dose-response studies were done in which additional noradrenaline was infused at 500, 1000 and 2000 ng/kg/min, each rate for 1 h. Blood samples were taken before and during infusions for measurement of haematocrit and plasma concentrations of noradrenaline, active renin, angiotensin II, aldosterone, sodium and potassium. Fourteen-day infusion of noradrenaline at 125 ng/kg/min did not raise blood pressure significantly though infusion at 250 ng/kg/min did, but for the first week of infusion only. Heart rate decreased significantly at both rates. Arterial pressure fell markedly and significantly on stopping infusion. Mean plasma concentrations of renin, angiotensin II and aldosterone tended to be lower during prolonged infusion of noradrenaline, but only the fall of renin during the second week was significant in one group of dogs. Noradrenaline at higher rates significantly raised blood pressure and increased plasma concentrations of renin and angiotensin II. Plasma aldosterone concentration did not rise significantly, perhaps because plasma potassium concentration decreased; in support of this theory changes of plasma aldosterone correlated with changes of plasma potassium but not with changes of angiotensin II. The rise in arterial pressure during dose-response studies was related to the increase of plasma noradrenaline. Prolonged infusion of noradrenaline did not alter the dose-response relation between plasma noradrenaline concentration and arterial pressure.