Urinary excretion of sodium, calcium, and magnesium has been measured after single oral doses of hydrochlorothiazide (100 mg) and acetazolamide (500 mg) in unselected patients with calcareous renal stone formation and in normal control subjects. With hydrochlorothiazide, 36 stone formers had significantly greater increments in sodium (P less than 0.01), calcium (P less than 0.05), and magnesium (P less than 0.05) excretion than 20 normal subjects. With acetazolamide, 13 stone formers had a smaller increment in sodium excretion (P less than 0.05) than 10 normal subjects. The abnormal responses to both diuretics were most marked in the patients with hypercalciuria during fasting. These data suggest that the tubular handling of sodium, magnesium, and calcium may be abnormal in patients with calcareous renal stones and are consistent with the presence of a defect in proximal-tubular reabsorption of fluid and electrolytes that may be partly offset by increased reabsorption in the distal nephron.