If you're going to try it out, get the large ones made for ferrets. Some rats actually do like to run around in them (just like some like to swim). If your rats don't like it, take them out and just pop it in their cage with the lid off (so there's a large opening) and put some comfy blankies in it, and let them start sleeping in them. I've seen so many adorable ratties sleep in them... they'd prefer that over anything, from what I've heard though.