Poly(N-isopropylacrylamide) (PIPA) was synthesized by radical polymerization with 2,2’-azobisisobutyronitrile (AIBN) as an initiator and 3-mercaptopropionic acid (MPA) as a chain-transfer reagent in methanol (MeOH) at 70 degrees C for 7 h. The resultant PIPA was grafted to polyallylamine hydrochloride (PAlAm.HCl) by amide formation under the influence of water-soluble carbodiimide 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC). The graft polymer was made into microspheres (MS) by chemical crosslinking. The pH-responsive drug release of the graft polymer microspheres was examined by releasing phenobarbital natrium (PN), which was carried on the microspheres by physical adsorption. A dynamic dialysis technique was used in the drug-release experiment and the drug-release-rate constants reflecting the drug release characteristic of polymer microspheres were obtained by constituting a mathematical model. The results indicated that the homopolymer PAlAm microspheres and the copolymer PAlAm-g-PIPA microspheres are both pH responsive to release PN and that in the neutral pH condition the release rate is the slowest.