This paper describes the design and development of a mechatronic testbed facility which allows high-fidelity and low-cost testing of the joint prototypes of a manipulator in a highly flexible environment. The testbed system consists of a set of load motors whose shafts are connected to those of the joint prototypes through a set of torque transducers. A controller modifies the simple dynamics of the load motors to match the nonlinear and coupled dynamics of the manipulator links. This is made possible by incorporating the measurement of joint angles, velocities, and joint torques, as well as the dynamics model of the manipulator links in a composite feedforward/feedback loop. The stability of the overall system and the fidelity of the load emulator are investigated. The testbed system is constructed to permit testing in a space-like thermal/vacuum environment. Thus, the facility is particularly useful for development and validation of new joints and actuators for space manipulators. The system is experimentally validated by comparing trajectories of the joint angles, velocities, and torques of a set of joint prototypes obtained by installing them first on an actual robot, and then on the mechatronic testbed