Hydrogels, composed of poly(N-vinyl-2-pyrrolidone) and crossed polyacrylamide, were synthesized and the release of vitamin Bit from these hydrogels was studied as a function of the degree of crosslinking and pH of the external swelling media. The three drug-loaded hydrogel samples synthesized with different crosslinking ratios of 0.3, 0.7, and 1.2 (in mol %) follow different drug-release mechanisms, that is, chain relaxation with zero-order, non-Fickian and Fickian, or diffusion-controlled mechanisms. To establish a correlation between their swelling behavior and drug;release mechanism, the former was studied by the weight-gain method and, at the same time, the concentration of the drug released was studied colorimetrically, Various swelling parameters such as the swelling exponent n, gel-characteristic constant it, penetration velocity v, and diffusion coefficient D were evaluated to reflect the quantitative aspect of the swelling behavior of these hydrogels. Finally, the drug-release behavior of the hydrogels was explained by proposing the swelling-dependent mechanism. (C) 2000 John Wiley & Sons, Inc.