Rat-baiting is a blood sport involving the baiting of rats in a pit. It was a popular sport until the beginning of the 20th century. Rat-baiting involved filling a pit with rats and then placing bets on how long it would take for a terrier to kill them all.

Billy, the celebrated Rat Killing Dog, London, circa 1823

The officials included a referee and timekeeper. Pits were sometimes covered above with wire mesh or had additional security devices installed on the walls to prevent the rats from escaping. Rules varied from match to match.

In one variation there was a weight handicap for each dog. The competing dog had to kill as many rats as the number of pounds the dog weighed, within a specific preset time. The prescribed number of rats was released and the dog was put in the ring. The clock started the moment the dog touched the ground. When the dog seized the last rat, his owner grabbed it and the clock stopped.

Rats that were thought still to be alive were laid out on the table in a circle before the referee. The referee then struck the animals three times on the tail with a stick. If a rat managed to crawl out of the circle, it was considered to be alive. Depending on the particular rules for that match, the dog may be disqualified or have to go back in the ring with these rats and kill them. The new time was added to the original time.

A combination of the quickest time, the number of rats and the dog's weight decided the victory. A rate of five seconds per rat killed was considered quite satisfactory; fifteen rats in a minute was an excellent result.

Cornered rats will attack and can deliver a very painful bite. It was not uncommon to see a ratter left with only one eye in its retirement.