The potassium sparing effect of 5 mg of amiloride (A) given either alone, or in combination with 50 mg of hydrochlorothiazide (HCTH) was evaluated in 23 patients with essential hypertension on a daily diet containing 70 mEq of K and 130 mEq of Na. Two studies were carried out. In the first, patients received HCTH alone (2 months) followed by A alone (2 months) and the two combined (6 months). The results of this study demonstrate that A is capable of correcting and preventing the loss of serum and total body potassium (TBK) and of reducing the increase of serum uric acid induced by HCTH. In the second study, each patient received from the beginning A + HCTH for 12 months; no statistically significant changes for serum and total body potassium and for serum uric acid appeared in any patient after six and twelve months. This study proves that A is capable of clinical potassium sparing effect in hypertensives and that this effect is a long lasting one.