Eudragit RS, RS-RL, RL and S microcapsules containing ketoprofen were prepared by the solvent evaporation process in oil phase. The sustained release effect of these microcapsules and Oruvail, the representative commercial product of ketoprofen, was evaluated by the pH shift dissolution method and in beagle dogs, respectively. The dissolution patterns of ketoprofen from Eudragit RS, RS-RL and RL microcapsules were independent of the pH of the dissolution medium, and its dissolution rate increased with increasing content of ketoprofen in microcapsules. But the dissolution pattern of ketoprofen from Eudragit S microcapsules and Oruvail was found to depend on the pH of the dissolution medium. The rank order of the dissolution rate of ketoprofen from Eudragit RS, RS-RL and RL microcapsules containing 30 and 40 per cent (w/w) ketoprofen was sufficiently clear as to enable prediction of the relative bioavailability of ketoprofen from these microcapsules. In vivo evaluation using beagle dogs, sustained release effects of Eudragit RL and Eudragit S microcapsules containing 30 per cent (w/w) ketoprofen and Eudragit RS-RL microcapsules containing 40 per cent (w/w) ketoprofen were almost the same as or slightly superior to that of Oruvail.