Abstract Ammonium heparin injections increased renal excretion of sodium, water and inorganic phosphorus in thymectomized, but not in sham-thymectomized or unoperated rats drinking tap water. Heparin also increased the excretion of potassium in sham-operated animals and of chloride in all surgical groups. When the animals drank 1% NaCl, heparin increased excretion of sodium, potassium, chloride, and water in shamthymectomized as well as thymectomized rats. In studies on rats drinking saline during July, August, September, October, and January, thymectomized-adrenalectomized rats excreted less sodium, potassium, chloride, and water, but more inorganic phosphorus, than did sham-thymectomized-adrenalectomized rats, and excretion of sodium, potassium, and chloride was increased with heparin. During February and March, the increase in electrolyte excretion was greater in thymectomized-adrenalectomized than in sham-thymectomized-adrenalectomized rats, and heparin tended to bring values for thymectomized-adrenalectomized rats closer to those of sham-thymectomized-adrenalectomzzed controls. Phosphorus excretion was not influenced in adrenalectomized rats by either timing of the experiments or by injections of heparin. It is concluded that heparin exerts influences on electrolyte excretion which are not dependent upon suppression of aldosterone secretion, and that removal of the thymus gland increased the sensitivity to heparin.