Purpose: The purpose of this study was to quantify and compare the retentive forces of five different prosthetic attachment designs on mandibular overdentures retained by two or three implants. Materials and Methods: Two photoelastic acrylic resin models of an edentulous mandible were cast. In one model, three implants were aligned parallel to each other and oriented perpendicular to the occlusal plane. The distal implants were placed in the canine regions, with an interimplant distance of 22 mm. In the other model, two implants were placed using the same approach, with an interimplant distance of 22 mm. Five retention mechanisms were studied on both models: a bar with yellow clips, a milled galvanoformed bar, a bar with two clear distal locator attachments, a bar with two distal Ceka attachments, and a bar with clear locator attachments. Ten dentures were fabricated in clear acrylic resin. The specimens were loaded in tension at a crosshead speed of 50 mm/min in a universal testing machine. Five hundred forty cycles of repeated insertions and removals were performed to simulate 6 months of clinical function. Results: Initial retention forces ranged from 23.33 to 54.32 N. Retentive values of the clear locator attachments on the three-implant model were higher than those for other attachments. Conclusion: All designs demonstrated a decrease in retention from the initial testing to the final pull-out test. This decrease was significant for all designs. Keywords: attachment systems, implant-retained mandibular overdentures, retention