OBJECTIVE To develop an HPLC method for determination of the plasma concentration of aristolochic acid I (AA I ) and aristolochic acid II (AA II) and study their pharmacokinetics in rats. METHODS The plasma samples were extracted with acetonitrile. The analysis involved a C18 column as stationary phase and methanol, water and acetic acid as mobile phase. The flow rate was 1.0 mL min(-1), the UV detection wavelength was 315 nm. After a single intravenous dose of 5 mg kg(-1) AA in rats, the pharmacokinetic parameters were estimated. RESULTS The calibration curve of AA I was linear over the range from 0.056 mg L(-1) to 56.3 mg L(-1) with a correlation coefficient of 0.9997. The mean recovery rate was 88.7%. The RSD of within-day and between-day were all less than 8%. And the calibration curve of AA II was linear over the range from 0.192 mg L(-1) to 11.52 mg L(-1) with a correlation coefficient of 0. 998 9. The mean recovery was 85.8%. The RSD of within-day was less than 3% and between-day was less than 10%. The main pharmacokinetic parameters were estimated to be as follows: CL = (0.010 +/- 0.003) L min(-1) kg(-1), t(1/2alpha) = (8.2 +/- 1.7) min, t(1/2beta) = (79.6 +/- 28.5) min for AA I; CL = (0.003 +/- 0.001) L min(-1) kg (-1), t(1/2alpha) = (56.7 +/- 38.1) min, t(1/2beta) = (209.3 +/- 37.9) min for AA II. CONCLUSIONS The established HPLC method is simple and sensitive to determine the concentration of AA I , AA II and the metabolite of AA I in rat plasma. From the result of animal's test, we can find that AA I was quickly eliminated from plasma, the elimination of AA II and Aristololactam-the metabolite of AA I - were slower than that of AA I.