I purchased a med. & a large ball for my 8-mn old border collie. She has eaten/destroyed at least $150 worth of toys, balls, etc. since I got her. If something lasted 3 hrs, it was a miracle. She has had these balls for over a week & there is not even a toothmark in them. They are incredibly durable & heavy but they still roll & bounce well. She still prefers a tennis ball that she can descimate in a matter of minutes but she's not adverse to her new toys!