Abstract: A movement-responsive system for conducting tests on freely-moving animals is disclosed. The invention includes a container for housing the animal, a mechanism for rotating the cage in response to rotational movement of the animal, and one or more test leads connected to the animal for performance of biomedical tests such as infusion, in vivo ultrafiltration, laser-doppler monitoring of blood flow, electrical stimulation, and in vivo microdialysis. The container is caused to rotate in the opposite (counter-rotation) direction of the sensed direction of rotation of the animal so that the one or more test leads do not become entangled, twisted, disconnected, or clamped. The rotating mechanism includes a simple, reliable sensor assembly including a pair of strategically placed, close-ended limit detectors which are activated by a triggering element, which is also a part of the sensor assembly.