A stability-indicating, high-performance liquid chromatographic method is presented for the simultaneous determination of hydrochlorothiazide and triamterene in two-component capsule formulations. An aliquot of the sample is dissolved in a mixture of acetonitrile and aqueous acetic acid, mixed with m-hydroxacetophenone as an internal standard, and chromatographed on octadecylsilane bonded to microparticulate silica using 80% acetate buffer (pH 5.0, 0.004 M)-15% acetonitrile-5% methanol as the mobile phase. The relative standard deviations are +/- 1.1-1.6% for hydrochlorothiazide and +/- 1.7-2.2% for triamterene for two formulations. Recoveries of the two drugs added to placebos ranged from 98.4 to 101.7%.