The purpose of this study was to investigate the effect of modification of clasp assemblies on connecting rigidity. Unilateral distal-extension removable partial dentures were constructed for this study with a simulation model. A cone-telescopic crown and 10 different clasps were used as direct retainers. The ten clasps included Akers clasp, double Akers clasp, Kratochvil type RPI clasp, and seven modified clasps. These modified clasps were made from three different types of conventional clasps to which a guide plate, enlarged bracing arm, positive rest with hook, and distal rest were added. Vertical loads were applied, and the stress on abutment teeth, load distribution, and dislodgement of the denture base were measured.<BR>The results were as follows:(1) the partial dentures retained by a cone-telescopic crown and clasps with a distal rest generated large vertical and distal stresses on the abutment teeth;(2) modified Akers clasps with an enlarged bracing arm and/or positive rest caused less load distribution but large vertical movement of the denture base;(3) stress characteristics of the abutment teeth and load distribution of the denture base of the modified double Akers clasp and modified RPI clasp with guide plate, enlarged bracing arm, and distal rest were similar to those of the cone-telescopic crown; and (4) the cone-telescopic crown and clasps with amesial rest caused less vertical displacement of the denture base.