A simple,selective,sensitive,precise,simultaneous high performance liquid chromatographic analysis of serum samples and commercial tablet formulation containing hydrochlorothiazide,olmesartan medoxomil and irbesartan are reported.Good chromatographic separation was achieved using a μ-Bondapak,C18 column (15 cm×4.6 mm,5μm),and a mobile phase consisting of acetonitrile-0.2%acetic acid aqueous solution(50:50,v/v)at a flow rate of 1.0 mL/min.The ultraviolet detector was set at a wavelength of 260 nm.Hydrochlorothiazide,olmesartan medoxomil,and irbesartan were eluted at 1.2,3.8,and 4.4 min.respectively.No extraneous materials were found to interfere.The method uses protein precipitation with acetonitrile for the preparation of serum sample.The linear ranges for hydrochlorothiazide,olmesartan medoxomil,and irbesartan were 6.25-18.75,20-60,and 75-225 ng/mL,respectively.The recoveries of hydrochlorothiazide.olmesartan medoxomil,and irbesartan in spiked samples were all greater than 98%,and their relative standard deviations were less than 2.0%.The limits of detection were 1,2,and 2 ng/mL for hydrochlorothiazide,olmesartan medoxomiJl,and irbesartan,respectively,and the limits of quantification were 3 ng/mL,which allow their determination at the expected serum concentration levels.