Kritter Krawlers www.petco.com - $18.99 (12.5" giant) Product Description: Giant exercise ball for small animals like ferrets, guinea pigs, and hedgehogs. As your pet runs inside the ball, he drives it around the room! Fun for you and your pet. Shatter-resistant plastic ball is well ventilated, with two openings for easy entrance and exit. RattyRat's Review: Through research, we have found that exercise balls are not appropriate for rats to use. According to one of our rat care books, It says that rats do not like it's unstable nature, and can become stressed and overheated in them. We've also heard people on various forums talking about them being a problem for their rats when they gave them a try. We returned ours before ever using it. We felt it was better to be safe than sorry. The smaller balls can make good sleeping dens, though, if propped in the corner of your rat's cage, with it's lid off.