Kong brand dog ball set. These balls combine two toys in one: the classic tennis ball and the sound of the whistle when you bite them. Unlike ordinary tennis balls, the plush covering is specially designed to be non-toxic to dogs. These balls are of medium hardness so that the dog can bite them, but at the same time they are resistant so that they can withstand the bite of the animal. Available in four sizes: XS, S, M and L. On this same page, below, you will find the specific SIZE GUIDE for this product for more information on measurements. IMPORTANT NOTE: Please measure your dog's mouth as wide as possible to choose the right size for him. It is very dangerous for a dog to play with balls that it could swallow due to the risk of suffocation.