A novel injectable thermogelling poly(ester-anhydride) co-polymer, methoxy poly(ethylene glycol)-poly(sebacic acid-D,L-lactic acid)-methoxy poly(ethylene glycol) (mPEG-poly(SA-LA)-mPEG) triblock co-polymer, was prepared by melt-condensation polymerization. The synthesized triblock co-polymer was characterized by FT-IR and (1)H-NMR. The aqueous solutions of mPEG-poly(SA-LA)-mPEG underwent sol-gel precipitation transition when the temperature was increased from 20 to 70 degrees C, depending on the concentration of the polymer. 5-FU, as the model drug, was mixed into the gel in a low-viscous sol state at room temperature. About 63 wt% of the loaded 5-FU could be released in vitro from the gel over 72 h at 37 degrees C. Subcutaneous injection of 25 wt% mPEG-poly(SA-LA)-mPEG aqueous solution resulted in the formation of a in situ gel depot in a rat model, which sustained for longer time than that of Pluronic F-127 aqueous solution. The biodegradable thermogelling mPEG-poly(SA-LA)-mPEG triblock co-polymer is believed to be a promising candidate for drug-delivery applications.