Shelters use a number of ways to euthanize, but most veterinarians use an injection of phenobarbital, a sedative that is given in large amounts for the euthanasia process. The drug can be administered orally or intravenously.
Phenobarbitol is similar to drugs that humans receive as an anesthetic before a surgery. Once the drug is administered, the human or animal receiving it just falls asleep. Once the cat is asleep, the veterinarian administers more and more of the drug. The cat feels nothing during this time. As the cat continues to receive more phenobarbitol, its respiration begin to decrease dramatically, until the animal completely stops breathing, at which point the cat's heart stops beating.
The entire process takes roughly 30 seconds to one minute after the initial sedation depending on the size and condition of the animal. Almost no pain associated with the entire process, other than the initial pain of the needle stick.
Local laws determine what happens to a cat's body after it is deceased. Some local laws allow the pet owner to take his pet's body home and dispose of it as he sees fit. Other laws prevent pet owners from removing the body from the veterinarian's care.