A novel triblock copolymer for use in an injectable pH- and temperature-sensitive hydrogel is synthesized by conjugating poly(amidoamine) (PAA) to poly(ethylene glycol): poly(amidoamine)-poly(ethylene glycol)poly(amidoamine) (PAA-PEG-PAA). The polymer was characterized with H-1 NMR and gel permeation chromatography in the diluents CDC13 and CHC13, respectively. The PAA block acts as a pH- and temperature-sensitive block. The PAA-PEG-PAA copolymer in aqueous solution (12.5 wt %) underwent a sol-gel transition as a function of pH and temperature. After injection into a rat, the copolymer solution (12.5 wt %) was immediately changed to a gel.