It's pretty unusual for a rat to even get fat in the first place, as long as you give them a wheel and an interesting environment. I always let my rats run loose for most of the evening too, back when I kept them. You just open the door, they scatter, and when it's time to put them up, you shake a bag of peanuts and they all come running to get a peanut apiece and then back into their enclosure.Truly a shame to waste this amazing drug on rats...