Aqueous solutions of poly(ethylene glycol‐b‐[DL‐lactic acid‐co‐glycolic acid]‐b‐ethylene glycol) (PEG‐PLGA‐PEG) triblock copolymers form a free‐flowing sol at room temperature and become a gel at body temperature. In this study, in situ gel formation was investigated in rats. Upon subcutaneous injection of 33 wt % aqueous solutions of PEG‐PLGA‐PEG triblock copolymer into rats, transparent gels were observed. The gel showed good mechanical strength and the integrity of gels persisted longer than 1 month. The gel underwent degradation by hydrolysis and turned opaque. Degradation study showed preferential mass loss of PEG‐rich segment from the in situ formed gel. Number average molecular weight determined by gel permeation chromatography decreased from 3300 to 1900 and 30% mass loss was observed over 1 month. © 2000 John Wiley & Sons, Inc. J Biomed Mater Res, 50, 171–177, 2000.