pH-sensitive co-polymeric hydrogels consisting of N-vinyl-2-pyrrolidone and acrylic acid were synthesized and their swelling and network parameters were evaluated. The gels were also characterized by thermogravimetric analysis and Fourier Transformation Infrared Spectroscopy. The gels exhibited nearly 47.8 +/- 4.9% swelling in the medium of pH 1.2 while nearly 2164.6 +/- 21.8% swelling was observed in the phosphate buffer medium of pH 6.8 at 30 degreesC. The swollen gel underwent sharp volume-transition in solutions containing Ni2+ and Ca2+ ions which was explained on the basis of monovalent-divalent ion exchange phenomenon taking place between gel phase and solution phase. The hydrogel, loaded with model drug vitamin B-12, demonstrated nearly 8.6 +/- 2.1 % and 83.2 +/- 4.8% release in the media of pH 1.2 and 6.8, respectively, as determined by traditional dissolution test. However, in order to incorporate in vivo GI conditions such as acidic pH and high water content in the stomach, low water content and presence of semi-solid mass in the large intestine, a new test model, called flow through diffusion cell was also used to study the drug release behavior of the device. The two approaches yielded almost different release profiles. (C) 2004 Elsevier B.V. All rights reserved.