I fed my 120g male butter, he's somewhere between 18 and 24 inches, a 3 week old rat 2 days ago. he was a little reluctant at first, but after about 15 minutes of him sitting back and watching it run around, he grabbed it and ate it without any problems. the rat weighed about 40g. so i would ask the pet store how old their rats are. 3 week old rat should be fine