A high-performance liquid chromatography-tandem mass spectrometric method (LC/MS/MS) has been developed and validated for the determination of pegylated liposomal doxorubicin and its metabolite doxorubicinol in rat plasma. One hundred microliters plasma samples were treated with Triton X-100 to immediately disperse the liposome. Then the samples were extracted by a single methanol:acetone protein precipitation step in the presence of additional 50microL of 70% (w/v) zinc sulfate, and subsequently analyzed by LC/MS/MS using positive turbo-ion spray ionization mode operating the instrument in the multiple-reaction-monitoring (MRM) mode. The related compound daunorubicin was used as internal standard. The validated concentration ranges were from 20 to 8000ng/mL for doxorubicin and from 0.05 to 20.0ng/mL for doxorubicinol. An effective LC-MS/MS method was developed to quantify trace amount of doxorubicinol with little interference from doxorubicin. The autosampler carryover was minimized from 285 to 10.5% by increasing the washing times of the valves when the used pentafluorophenylpropyl HPLC column had no contribution to the carryover. The relative matrix effect from six unique lots was absent for both compounds. Results obtained from the GLP validation study demonstrated good accuracy (85-110%) and precision (CV less than 14%) across the calibration ranges for both compounds. This method was applied to study the pharmacokinetic profiles of doxorubicin and doxorubicinol in rats after a single dose administration of Stealth-49 liposomal doxorubicin HCl. The mean AUC value for doxorubicinol was found to be only 0.011% of that of doxorubicin.