A new method has been developed for the quantitative estimation of the diuretic and antihypertensive agent, hydrochlorothiazide, in human serum and urine. The drug is extracted into ethyl acetate from biological material and then back-extracted into sodium hydroxide solution. This aqueous solution is chromatographed on a reverse phase system, using a µBondapak C18 column and methanol–0.01 M sodium dihydrogen phosphate as eluent and detection at 271 nm. The recovery from serum was 98.2 ± 3.7% (mean ± SE, n= 9) over the concentration range of 200-800 ng/ml. In urine, the recovery was 91.5 ± 2.5% (mean ± SE, n= 8) over the range of 20-100 µg/ml. No interference was observed in this assay from the following drugs which may be administered concurrently during hydrochlorothiazide therapy: azathloprine, chlorthalidone, guanethidine, methyldopa, minoxidil, prednisone, and spironolactone.