Oligodcoxyaucleotide-doxorubicin conjugate is a novel modified oligodeoxynucleotide (ODN) to inhibit the expression of mdrl gene. The present study was undertaken to determine the stability of the conjugate in virto and its pharmacokinetics in vivo using a reversc-phase HPLC assay. The method employed a Sort C18 reverse phase column combined with a C8 prc-column and a linerar gradient elution with acetonitrile containing 0.1M aqueous triethylammonium acctate, pH7.0 Detection was carried out using a UV-didoe array detector at 254 and 480nm. Minimum sensitivity of ～0.15μg/mL in plasma and 0.1μg/mL in PBS of hte conjugate was achieved. After incubation in 10% activated fetal calf serum for 24 hours, 15.8% of the conjugat was degraded. As a comparison however, 97.5% of the control ODN was degraded witin the same incubation time. The phamacokinetics stuies show that the halif-lives of the conjugate is about 8 hours, 4 times longer than ODN as a conro. Assay validaton studies revealed that the method is accurate, reproducible for dcterminatin of the conjugate, and can be used for a pharmacokinttic study of the conjugate.