To elucidate the mechanisms involved in increased parathyroid function in primary aldosteronism (PA), we evaluated the effects of an intravenous NaCl load on Ca metabolism and plasma level of intact parathyroid hormone (PTH) in patients with PA compared with that in patients with essential hypertension (EH). Sixteen PA patients and 16 EH patients who were well matched for age, gender, body mass index, renal function, and systolic (SBP) and diastolic blood pressure (DBP) were examined. In each subject, after 6 days of a controlled intake of Na, K, and Ca, isotonic saline was infused at a rate of 500 mL/h for 4 h. At baseline, in spite of similar BP values and urinary Na excretion (U[Na]V), urinary excretion of Ca (U[Ca]V) and PTH were higher in the PA group than in the EH group. In both groups, the NaCl load caused a decrease of serum ionized Ca (Ca2+) and an increase in PTH, U(Na)V, and U(Ca)V. However, these changes were significantly greater in the PA group. The increased baseline U(Ca)V in PA could be due to reduced reabsorption of sodium in aldosterone insensitive tubular sites, as a result of the "escape phenomenon." The increased U(Ca)V may explain the higher basal PTH in PA patients, which is needed for maintaining a normal Ca2+. The greater changes in the Ca2+/PTH profile elicited by the saline load in PA patients are apparently due to a higher calciuretic response following a more exaggerated natriuresis in PA.