An improved HPLC method for analysis of methamphetamine (MAMP) and its metabolites in plasma was established, which features: high and reliable extraction recovery without loss of the analytes during solvent evaporation; no need for sample derivatization, and simultaneous analysis of MAMP, its N-demethylation and 4-hydroxylation metabolites. The major modification on the previous extraction procedure is addition of HCl to ethyl acetate extract prior to evaporation, resulting in significant increases in recovery, reproducibility and sensitivity. HPLC analysis is performed with an isocratic system on a C6 column. Samples are eluted with a mobile phase of acetonitrile-10 mM phosphate buffer containing 0.1% triethylamine (pH 6) at a flow rate of 1.0 ml/min, and monitored by a UV detector at a wavelength of 214 nm. All the analytes elute within 24 min. This method was successfully applied to the analysis of MAMP and its metabolites in rat plasma after MAMP dosing.