A simple and sensitive high-performance liquid chromatographic method for quantitation of hydrochlorothiazide in human plasma was developed and bioavailability parameters of hydrochlorothiazide were assessed in Korean healthy male volunteers. Caffeine was used as an internal standard. Hydrochlorothiazide and internal standard in plasma sample were extracted using tert-butylmethylether (TBME). A centrifuged upper layer was then evaporated and reconstituted with mobile phase of acetonitrile-25 mM phosphate buffer (20/80, pH 2.5). The reconstituted samples were injected into a Luna C18 column $(250{ imes}4.6;mm,;5{mu}m)$ at a flow-rate of 1.0 ml/min. The wavelength was set at 230 nm and no endogenous substances were found to interfere, A linear relationship for hydrochlorothiazide was found in the range of $10{sim}300;ng/ml$. The lower limit of quantitation (LLOQ) was 10 ng/ml with acceptable precision and accuracy. Assayed in plasma, the intra- and inter-day validation for all coefficients of variation (R.S.D.%) were found less than 15%. Main pharmacokinetic parameters of 50mg of hydrochlorothiazide were revealed as follows: $AUC_t;1761{pm}509.0;ng{cdot} hr;ml,;C_{max};296.5{pm}95.5;ng/ml,;T_{max};1.94{pm}0.85hr,;K_{el};0.12{pm}0.04;hr^{-1};and T_{12};6.81{pm}2.92;hr.;C_{max};and;T_{max}$ were in accordance with the values $(270{sim}350;ng/ml;and;1.9{sim}2.7;hr)$ of Caucasian.