In this study, conventional and sustained-release suppositories of ketoprofen (KP) were prepared and the effects of suppository bases and the inert matrix material (hydroxypropylmethylcellulose phthalate, HP 55) on in vitro release of ketoprofen suppositories were investigated. Suppositories containing 100 mg ketoprofen were prepared by the fusion method. Witepsol H15, Massa Estarinum B, and polyethylene glycols (PEG) were used as examples of hydrophobic and hydrophilic bases, respectively. Sustained-release suppositories of KP were prepared by using HP 55. Weight variation, content uniformity, breaking (hardness) and melting range tests were then conducted on these formulations. In vitro release and diffusion rate tests were also carried out according to the USP XXII basket method and the Muranishi method, respectively. The results show that the rate of release of KP is very slow for Witepsol H15 and Massa Estarinum B bases. However, KP was released very rapidly from the PEG bases in the conventional suppositories. On the other hand, HP 55 might be useful as a vehicle for sustained release preparations of ketoprofen in suppository form. It was shown from the kinetic assessment of release data that the best fit was achieved with zero-order kinetics.