The gatifloxacin mesylate concentration in human plasma and urine was determined by HPLC on Zorbax SB C18 anal. column with H3PO4-MeOH-NEt3 (64.7:35:0.3, pH 3.0) as mobile phase and the detection at Vex 295 nm and Vem 495 nm.A simple accurate and reproducible procedure with HCl was employed for pretreatment of plasma samples and the urine sample was directly injected.The internal standard was ciprofloxacin.The linearity of plasma and urine concentration of gatifloxacin mesylate was between 0.015 6-1.0 mg/L-1 and 0.434-111.11 with r = 0.999 7, resp.The absolute recovery of gatifloxacin mesylate in plasma and urine was 78.01-117.1% and 96.73-106.5% and the relative recovery was 88.7-94.6%, 96.70-110.0%, the within day and between days derivation (relative standard deviation) in the plasma and urine assay was <13% and <7.4%, resp.The method was simple, quick with the high sensitivity and could be used to study the pharmacokinetics of gatifloxacin mesylate.