Six healthy young volunteers were fed during two period of 9 days each by a strictly constantly liquid formula diet (Fresubin), on an ambulator basis. In a single blind cross-over randomized study, they were given either hydrochlorothiazide, 50 mg p.d., or a placebo during the first seven days of the two periods. The diuretic did not induce significant changes of either magnesemia or magnesiuria. In addition, the magnesium excretion after a challenging intravenous magnesium load remained unchanged. However, the thiazide normally increased the urinary excretion of sodium and potassium. It also induced hypocalciuria and hypercalcemia. The urinary excretion of cAMP and oxalic acid remained stable.