Abstract: The present invention relates generally to the development of pharmaceutical compositions which provide for sustained release of biologically active polypeptides. More specifically, the invention relates to the use of pH/thermosensitive biodegradable hydrogels, consisting of a A-B di block or A-B-A tri block copolymer of poly(d,l- or l-lactic acid) (PLA) or poly(lactide-co-glycolide) (PLGA) (block A) and polyethylene glycol (PEG) (block B), with ionizable functional groups on one or both ends of the polymer chains, for the sustained delivery of biologically active agents.