A method that describes the determination of the in vitro release of ketoprofen from gels was suggested. The experimental system of the method consists of a Franz diffusion cell, which contains a pH 7.4 phosphate buffer as a receptor medium, and a $70;{mu}m$ mesh woven nylon membrane as a diffusion barrier. Under the given condition of the system, the diffusion of ketoprofen across the membrane was rapid enough that the apparent release profile of ketoprofen obtained from the present method could represent the release of the drug from gel preparations. The release of ketoprofen in the present method was reproducible, and the rate increased in proportion to the concentration of ketoprofen in the gel. These suggest that the present method is applicable to the quality evaluation of gel preparations containing ketoprofen.