Bioequivalence(BE) test of commercially available sustained release tablets of diltiazem hydrochloride(DTZ) was performed to give some guidelines to BE test in korea in case of which drugs with low oral bioavaiiability(BA) due to substantial first-pass hepatic loss form pharmacologically active metabolites. In such cases, the pharmacologic activity after oral administration is greater than anticipated from BA data, based on chemical assay of drug alone. Therefore, this paper explores the use and meaning of area under the plasma concentration-time(AUC) data of parent and its metabolites to access BA if sustained release tablets. Normal healthy male volunteers(n=14) were randomly divided into 2 groups, and sustained release reference$(Herbesser^{(R)})$ and test$(Herben^{(R)})$ tablets of DTZ-30mg were given orally by balanced two-period cross-over dosing schedule. The plasma concentration of DTZ and and its active metabolite, desacetyldiitiazem(DAD), were determined by high performance liquid chromatography, and, $AUC_{DTZ},;AUC_{DAD},;AUC_{DTZ+DAD},;C_{max};and;T_{max}$ obtained. Analysis of varlance(ANOVA) showed that $AUC_{DTZ};and;C_{max}$ passed the standard $(alpha=0.05,;1-etageq0.8,;Deltaleq0.2)$ of BE test of korea, but $AUC_{DAD}$ was not satisfied from the standpoint of power. On the other hand, $AUC_{DTZmidDAD}$ may be more avaliable than $AUC_{DAD}$ from the standpoint of statistics and pharmacologic equivalence.