Poly-L-lactic acid (MW 6000) microspheres (PLA-MS) containing 5-fluoro-2'-deoxyuridine (FUdR) or four ester prodrugs of FUdR were prepared and examined with regard to the in vitro release kinetics. The incorporation efficiency of the lipophilic prodrugs into the PLA-MS was higher than that of FUdR or the hydrophilic prodrugs. The release of the lipophilic FUdR prodrugs from PLA-MS was sustained as compared with that of FUdR from PLA-MS. The release kinetics of the FUdR prodrugs appears to fit the Higuchi, Baker, and Lonsdale model in the early stage of the release process. The slope of the Baker and Lonsdale plots of the release of divaleryl-FUdR from PLA-MS decreased as the initial drug loading was decreased. The order of the release rates of FUdR prodrugs from PLA-MS with the same prodrug content was similar to that of the water solubilities of the prodrugs. These results suggest that the ester prodrugs could be released from PLA-MS by diffusion through water-filled capillaries or a series of pores rather than by diffusion through the PLA matrix.