1. Sodium, calcium and magnesium reabsorption by the renal tubules are interrelated. 2. To determine whether the tubular transport of calcium and magnesium were influenced directly by adrenocortical hormones, clearance studies were carried out in normal adults before and after the administration of either a placebo, aldosterone or cortisol. 3. During the placebo studies urinary sodium and chloride excretion increased while potassium and net acid excretion were unaltered. 4. Following aldosterone infusion, sodium and chloride excretion fell while potassium and net acid excretion rose. 5. After cortisol infusion sodium excretion also fell but was accompanied only by a rise in potassium excretion. 6. Despite the observed changes in sodium excretion, neither placebo, aldosterone nor cortisol administration had any effect on the serum ultrafiltrate concentrations, rates of glomerular filtration or urinary excretion rates of calcium or magnesium. 7. We conclude that aldosterone and cortisol have no direct effect on renal tubular reabsorption of calcium or magnesium.