The pH-responsive swelling and release behaviors of anionic P(MAA-co-EGMA) hydrogel microparticles having various MAA and EG contents were investigated as a biological on-off switch for the design of an intelligent drug delivery system triggered by external pH changes. When DC was used as a dispersion stabilizer, well-dispersed hydrogel microparticles having an average diameter of approximately 4 mu m were obtained. There was a drastic change of the equilibrium weight swelling ratio of P(MAA-co-EGMA) hydrogels at a pH of around 5, which is the pK (a) of PMAA. When the MAA content in the hydrogel increased, the swelling ratio increased at a pH above 5 due to the more electrostatic repulsion between the charged groups of MAA. The P(MAA-co-EGMA) hydrogel microparticles showed a pH-responsive release behavior. At low pH (pH 4.0) small amounts of Rh-B were released while at high pH (pH 6.0) relatively large amounts of Rh-B were released from the hydrogels. The difference in the released amount of Rh-B from the hydrogels between pH 4.0 and 6.0 decreased when the MAA content in the hydrogels decreased, which means that the pH-responsive release behavior of the P(MAA-co-EGMA) hydrogel microparticles is closely related to the pH-responsive swelling property of the hydrogel.