The diuretic response of normal infants, 6 to 47 days of age, to single doses of mercaptomerin, chlorothiazide, acetazolamide, triamterene and spironolactone was studied by following urinary electrolytes, pH and osmolality. Peak diuresis occured two to four hours after drug administration, and because of compensatory mechanisms little change in urinary excretion was found if only 24-hour urines were studied. Mercaptomerin increased sodium excretion seven-fold, compared to three- to four-fold increases for the other diuretics. Control urinary Na:K ratios averaged 0.68 in infants compared to 2.8 for adults, and mercaptomerin produced the largest increase in this ratio. Qualitatively the response to diuretics is the same in newborn in the ages studied as it is reported to be for adults; no immaturity of the infant kidney in this regard was demonstrated.