The effects of calcium upon the renal concentrating and diluting mechanisms in man were studied in six normal subjects infused intravenously with 10 ml of a 10% calcium chloride solution, during sustained water diuresis and hydropenia. Four aatients with idiopathic diabetes insipidus undergoing water diuresis were studied in a similar fashion. The normal subjects during hydropenia had significant increases in both urinary excretion rates of sodium and Tc h 2o expressed as percentage of creatinine clearance. During water diuresis, the same subjects significantly decreased the values C h 2o and C h 2o expressed as percentage of creatinine clearance, four of them attaining Tc h 2o . The patients with diabetes insipidus behaved differently from the normal subjects, thus increasing significantly the C h 2o value. The mechanisms involved in the observed changes are discussed. The possible participation of calcium-induced ADH release in explaining the observed results is postulated.