Urinary sodium excretion was measured in adrenalectomized, euthyroid rats, within 4 hours after injection of aldosterone and/or thyroid hormone (T3). Rats which received both hormones excreted significantly more Na than rats injected with aldosterone alone. Reverse-T3 did not affect the antinatriuretic action of aldosterone. Natriuresis following injection of T3 alone was similar to that measured in the absence of exogenous hormones. These observations suggest an interaction between aldosterone and T3 on renal Na excretion.