Renal tubular function was examined in 5 adult patients aged 18-30 years with Bartter's syndrome associated with renal magnesium wasting and hypocalciuria. In the 3 patients studied during hypotonic saline diuresis, distal tubular fractional chloride reabsorption was lower than that reported in normal subjects. In response to a single intravenous dose of furosemide (40 mg), the increment in the excretion of sodium, chloride, and magnesium was equal to or greater than in normal subjects, while in 2 patients, in response to intravenous chlorothiazide (500 mg), the increment in sodium excretion was less than in normal subjects. Magnesium chloride infusion was undertaken in 2 patients in order to compare magnesium and calcium excretions at similar plasma magnesium levels in patients and in normal subjects. The patients exhibited magnesium wasting only at normal or low plasma magnesium levels, while calcium excretion was reduced in the patients at normal and elevated plasma magnesium levels. We conclude that in these patients the enhancement of renal magnesium reabsorption by hypomagnesemia is defective, and the hypomagnesemia is not the cause of the hypocalciuria. The tubule defect responsible for these abnormalities of magnesium and calcium excretion may be located beyond the side of action of furosemide, in the thiazide-sensitive segment of the distal convoluted tubule.