easiest and most cost effective way... Cooler + Dry Ice... place rat/mouse in bottem of cooler place some sort of rack or screen far enough above the rat/mouse so it isn't able to touch it, place dry ice ontop of said rack/screen... leave alone for a while CO2 fills the cooler the rat/mouse passes out and dies in its sleep.. as far as I've been told never done it but plan to if i have to euthanize, myself....