We have measured the secretion rate of norepinephrine (NE) in 6 euthyroid subjects, 6 hyperthyroid and 6 hypothyroid patients, infused at a constant rate (0.1 microC/kg/min) for 1 h with tritiated norepinephrine (New England Nuclear Inc.). Plasma NE concentrations were measured by a modification of the fluorometric method of Anton and Sayre. A significant linear relationship was observed between plasma NE and age in normal subjects. Accordingly, values for plasma NE have been corrected for age. Plasma NE concentration was 18.3 +/- 4.2 ng/100 ml (mean+/-SEM) in normal subjects compared with 17.5 +/- 3.3 ng/100 ml in hyperthyroid patients. There was a significant elevation of plasma NE (30.3 +/- 2.9 ng/100 ml) in hypothyroid patients (P less than 0.05). A significant elevation was observed in plasma NE secretion rates in hypothyroidism 4.62 +/- 0.98 microgram/kg/day (P less than 0.02) when compared to the control group (1.46 +/- 0.35 microgram/kg/day). No significant difference was observed between the hyperthyroid group (1.74 +/- 0.49 microgram/kg/day) and the controls. These data indicate that the plasma NE secretion rate is normal in hyperthyroidism, and is significantly elevated in hypothyroidism thereby explaining the higher plasma NE concentrations seen in hypothyroidism.