The urinary zinc/creatinine ratio has been measured in five groups of patients with essential hypertension and in a group of healthy controls. The first four groups of patients consisted of subjects being treated for at least three months with captopril alone, hydrochlorothiazide alone, captopril plus hydrochlorothiazide, or captopril and furosemide. The fifth group comprised hypertensive patients not on any medication. The first four patient groups exhibited significantly increased urinary zinc/creatinine ratios when compared to the control and untreated hypertensive groups, but in the two combination regimens there was little zincuria. It is suggested that both diuretics inactivate the zincuric effect of captopril by binding to its sulphydryl group within the tubular lumen.