Thyroid function, as evaluated by 131I biological half-life, thyroid secretion rate (TSR), and thyroid weight, was depressed by reserpine, α-methyl-p-tyrosine (α-MT), guanethidine and amphetamine. Tetrabenazine and e-aminocaproic acid (EACA) were without effect at the doses employed. A correlation was found between the degree of peripheral (cardiac) noradrenaline (NE) depletion, and the depressionof thyroid function. No such correlation was observed between the latter and central NE levels. It was concluded that peripheral NE storesplay a significant role in the maintenance of normal thyroid function.