Degrading hydroxyethylmethacrylate-grafted dextran (dex-HEMA) hydrogels generate a relatively sudden increase in osmotic pressure upon degradation into dextran solutions. This phenomenon is currently being examined as a possible means of developing a pulsatile drug-delivery system. Here a mathematical model based on scaling concepts is presented to describe this sudden increase in swelling pressure and to provide a framework for the rational design of pulsatile delivery systems based on this phenomena. The model provides a good fit to the swelling pressures measured for dex-HEMA gel/free dextran mixtures that simulate degrading dex-HEMA gels. (C) 2004 Wiley Periodicals, Inc.