The objectives of this study were to investigate the potential interactions between the model protein drug (bee venom peptide, BVP) and thermosensitive poly(dl-lactide-co-glycolide-b-ethyleneglycol-b-dl-lactide-co-glycolide) (PLGA-PEG-PLGA) copolymers and to examine the drug-copolymer interactions on the in vitro drug release and hydrogel degradation. The PLGA-PEG-PLGA copolymers were synthesized by ring-opening copolymerization of dl-lactide and glycolide with PEG as an initiator. Drug-copolymer co-precipitate blends were prepared and analyzed by Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) to characterize the specific interactions between drug and copolymer. For the better understanding the drug-copolymer interactions on drug release, insulin was selected for comparison. The release of the two protein drugs from the copolymer-based hydrogels and hydrogel degradation was studied at 37 degrees C under agitation. The results of FTIR and XRD indicated that the hydrogen bonding interactions existed between the NH group of BVP and CO group of the copolymers. The insulin and BVP released from the copolymer hydrogel over 15 and 40 days, respectively. The BVP-copolymer interactions retarded the BVP release rate and degradation of hydrogel, but did not significantly affect the biological activity of BVP. These results indicate that the drug-copolymer interactions need to be considered when attempting to use PLGA-PEG-PLGA hydrogels as sustained delivery carriers of protein or peptide drugs.