Urinary zinc excretion was studied in a randomized trial in 9 patients during treatment with bendroflumethiazide, chlorthalidone and hydrochlorothiazide and in another 9 patients during treatment with bumetanide, furosemide and triamterene. During treatment with the thiazides, the zinc concentration rose by 30% and the total amount of zinc excretion increased by 60%. In contrast, during treatment with the loop-diuretics, urine zinc concentration diminished and the total amount of zinc excretion increased much less than during therapy with the thiazides. With respect to the importance of zinc as an essential element in human metabolism and the frequency of diuretic treatment, the observed increased urinary losses of zinc deserve further attention.