Couplable first and second hemispheres form a hollow spherical chamber. An opening in one of the hemispheres has a door movable between a closed and an open position. The top of a base is in a concave configuration with a radius of curvature essentially equal to the radius of curvature of the spherical chamber and forms a recipient surface for receiving and rotatably supporting the spherical chamber. An animal within the spherical chamber may walk and run in place for exercising in a relatively restricted space.