Purpose/Objective(s): The purpose of the present work was to study the pharmacokinetics and biodistribution of drug-loaded nanodroplets in vivo in order to evaluate the efficiency of drug delivery to the prostate tumor. Materials/Methods: Poly{ethylene oxide}-co-poly {D, L-lactide} (PDLA) nanodroplets loaded with fluorescently labeled-paclitaxel (F-PTX) were synthesized using solid dispersion technique. Human prostate cancer, LNCaP cells were implanted orthotopically in the prostates of male nude mice. Tumorbearing mice (n Z 3) were systemically injected with 125ml of 0.1% F-PTX, 2% PDLA nanodroplets using tail vein. At chosen time points (30min; 2h; 6h; 24h) animals were anesthetized, bloodwas collected by eye bleed, then animals were sacrificed, tumor and vital organs (liver, spleen, lung and heart) were excised, cut in to halves and weighted. One cut portion of the organ was homogenized, incubated in lysis buffer and centrifuged. The lysates were read using fluorescence spectrophotometer (Excitation496nm, Emission524nm). Fluorescence readings were compared with values from the standard calibration curve. The other portion of the organ was frozen in liquid nitrogen and cut into sections for qualitative analysis using fluorescence microscope. Tumorbearingmice (nZ 3)were also injectedwith 0.1%F-PTX solution (fluorescent drug in free form) and mice with no injections were used as controls. Results: Quantitative analysis showed that for drug-loaded nanodroplets about 20% of the injected dose was found in tumor, 30 min after the systemic injection; which increased to 30% after 2h; then decreased to 27% after 6h and to 5%after 24h.When drug in free formwas injected, drug accumulation in tumor was 7%within 30mnt; 8%within 2h; 10%within 6h and 2%within 24h. Liver was the main organ that showed major drug accumulation for drug-loaded nanodroplets while spleen showed less. Least amount of drug concentration was in lung and heart. For drug in free form, liver and spleen showedmaximum drug concentrations while heart showed concentrations similar to that in tumor. Lung showed least drug accumulation. Qualitative analysis of microscopy images complemented quantitative data. Conclusions: In vivo studies showed that more drug was accumulated in the prostate tumor and less in vital organs when the drug was delivered using the drug-delivery system (nanodroplets) in comparison with drug delivered in the free form. These studies also helped determine optimum time-point after injection for exposing tumor (with accumulated drug-loaded nanodroplets) to high-intensity focused-ultrasound under MR guidance for treating prostate tumor effectively while minimizing drug related toxicities. Author Disclosure: R. Gupta: None. D. Cvetkovic: None. B. Wang: None. L. Chen: None. C. Ma: None.