CO2 sounds great to me. Even euthanizing a rat is tough. Since their veins are so small, the vets have to inject straight into the heart.
I didn't know CO2 would kill the rat, though. When I took my babies to be euthanized when they got old, the vet would put them under with CO2, but still had to inject.
The Maine Coon Mafia (litter mates)
Casey - Male Maine Coon, 3 years
Mak - Male Maine Coon, 3 years
The abandoned newcomer: Linus - Male Tabby, 1 year
Minnie - Female Tortie, RIP 9/3/93-8/18/09 (In Heaven, but never far from our hearts)
The Pooches... Best friends for life.
Miko - Male Chi/Terrier mix, 2 years
Mansa - Male Pit Bull, 1 year