This paper describes the biomechatronic design of a legged rat robot, to be used in experiments on rat behavior. The rat robot has been designed so as to possess leg and body motion similar to those of real rats and to show the capability of rat-like simple behaviors, like walking and pushing a lever. The rat robot has 4 legs with 3 DOF (2 active and 1 passive) for each leg and 1 additional DOF allowing robot spine torsion. A cable and pulley mechanism allows the two front legs of the robot to perform manipulation tasks like pushing levers and buttons in a smooth and natural way, as not only the geometry and mass of the legs are similar to those of real rats, but also because the cables act, from an anatomical and physiological point of view, like the triceps muscle with the knee tendon structure. An interaction experiment, using our rat robot and two different real rats (control and experimental subject) has been conducted to verify the capability of the robotic rat to interact and teach simple tasks to the real rat.