A blood microdialysis technique coupled with high-performance liquid chromatography was used to investigate the pharmacokinetics of unbound ketoprofen in rats after intravenous administration of a lipid-soluble ketoprofen derivate, ketoprofen isopropyl ester (KPI), loaded into lipid microspheres (LM) and ketoprofen solution. A microdialysis probe was inserted into the jugular vein of male Wistar rats. KPI-loaded LM or ketoprofen solution (24 mg/kg, i.v.) was then administrated via a femoral vein. Dialysate samples were analyzed using HPLC. The in vitro and in vivo recovery rate of the microdialysis probe was 30.42+/-0.74% (n=3) and 40.27+/-2.74% (n=3), respectively. The pharmacokinetic parameters for ketoprofen after intravenous administration of KPI-loaded LM and ketoprofen solution exhibited no statistically significant differences. The results of this pharmacokinetic study indicate that the microdialysis technique can be widely applicable to investigations of in vivo free-drug of microcarrier systems.