Abstract: A boundary responsive wheel lock assembly for a supermarket shopping cart, or the like, or for any other application where a wheeled device is to be boundary responsive, is provided, to prevent the removal of the cart from an authorized area. The assembly includes a roller which is mounted on the distal end of an arm which, in turn, is pivotally mounted to a bracket to be positioned adjacent to one of the wheels of the cart. A spring-loaded plunger is also mounted on the bracket, and the plunger is normally held in a cocked position by the arm as the roller moves along the floor of the supermarket or along the surface of the adjacent parking lot. The parking lot is surrounded by a barrier which extends, for example, one and a half inches above the ground. However, the flexibility of the assembly of the invention enables the assembly to respond to a variety of boundary heights.