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Amiloride is a potassium-sparing diuretic which has been advocated for the treatment of hypokalemic disorders. This agent was prospectively evaluated in hypokalemic patients with either primary hyperaldosteronism (ten patients) or Bartter's syndrome (five patients). Vital signs, electrolytes, and ambulatory hormonal studies were assessed during a control period and treatment period with amiloride therapy at 10 to 40 mg/day over two to 24 weeks. During the treatment period the systolic and diastolic blood pressure fell significantly in primary hyperaldosteronism but remained unchanged in Bartter's syndrome. In summary, amiloride therapy (1) increased plasma potassium in both diseases; (2) increased plasma renin activity (PRA) in primary hyperaldosteronism but decreased PRA in Bartter's syndrome; and (3) increased plasma aldosterone in both diseases. Since potassium is known to suppress renin production and stimulate aldosterone secretion, correction of the hypokalemia in this study probably accounts for the decreased PRA and increased plasma aldosterone observed in Bartter's syndrome. The increase in both PRA and plasma aldosterone in primary hyperaldosteronism, however, may be evidence of either a direct activation of the renin-aldosterone system or, alternatively, may be due to the mild natriuretic effects of amiloride. [ABSTRACT FROM AUTHOR]