Hydrogels based on pH-sensitive polymers are of great interest as potential biomaterials for the controlled delivery of drug molecules. In this study, a novel, pH-sensitive hydrogel was synthesized by poly(aspartic acid) (PASP) crosslinked with 1,6-hexanediamine and reinforced with ethylcellulose (EC). The loading and release characteristics of naproxen sodium (NS) were studied. The PASP-EC blend hydrogels had pH-sensitive characteristics and were strongly dependent on the pH value. The release kinetics for NS from the PASP-EC blend hydrogels and PASP hydrogel were evaluated in simulated gastric fluid (pH = 1.05) and simulated intenstinal fluid (pH = 6.8) at 37 degrees C. The results showed that the drug-loaded hydrogels were resistant to simulated gastric fluid, and hence, they could be useful for oral drug delievery. Compared with the PASP hydrogel, the PASP-EC blend hydrogels showed a lower release rate of NS in the same pH conditions. It was evident that the presence of hydrophobic groups (EC) retarded the release of NS and led to sustained release. The kinetics of NS release from the drug-loaded hydrogels conformed to the Korsmeyer-Peppas model. The release exponent of the model was 0.7291, which indicated multiple drug release. The PASP-EC blend hydrogels were biodegradable and pH sensitive; there would be a wide range of applications for them in controlled drug-delievery systems. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 327-336, 2009.