If you have access to Dry Ice (solid CO2) then this link will help you build a cheap CO2 tank that will put the rats to sleep and then kill them (most humane way to do it)


http://www.rodentracks.com/DIY_Co2_Chamber.html

If you don't have access to Dry Ice you can do the same thing with a paintball CO2 tank and hose but is more expensive to go this route. ??

Once you kill them (do 3-4 at a time) freeze them first. This way you won't be changing what your BP eats. If you do you run the risk of your BP not going back to F/T (doesn't always happen but it is a risk)

Let me know if you need more help.