1. Employing a precise and sensitive double-isotope derivative technique plasma noradrenaline and plasma adrenaline were measured in sixteen normal subjects, in eight patients with myxoedema and in ten patients with thyrotoxicosis. Venous blood was obtained while the subjects rested in the supine position.
2. In the normal subjects plasma noradrenaline averaged 0·22±SD 0·09 ng/ml and plasma adrenaline 0·05±SD 0·05 ng/ml. Plasma noradrenaline rose with increasing age.
3. In patients with myxoedema, plasma noradrenaline was increased approximately threefold. The difference between the normal subjects and the hypothyroid patients was more pronounced in the older age-groups.
4. Despite their elevated pulse rate patients with thyrotoxicosis showed significantly decreased plasma noradrenaline concentrations compared to the normal subjects.
5. In the hypothyroid and hyperthyroid subjects plasma adrenaline concentrations were not different from the values obtained in the normal subjects.
6. It is suggested that sympathetic nervous activity is decreased in thyrotoxicosis as a compensatory phenomenon to the direct effect of thyroid hormones on the cardiovascular system.