http://www.bu.edu/research/compliance/lacu/lasc/guidelines-policies/appendix20.shtml
CO2 Euthanasia of Rats and Mice

“Euthanasia" is a term meaning “good death". The goal of euthanasia is to provide a rapid, painless, stress-free death. Carbon dioxide overdose causes rapid unconsciousness followed by death. In adult mice and rats, unconsciousness occurs within 10 seconds and death occurs within 1-2 minutes of exposure to 100% CO2. (Note: Newborn animals and pups are highly resistant to CO2, and require at least 10 minutes of exposure to ensure death.) Exposure of mice and rats to CO2 without removing them from their home cage is a rapid and humane method of euthanasia, because the animals are not stressed by handling or being moved to a new environment.