General Pet Policy for most major car rental companies: Dogs of all sizes are allowed for no additional fee and no restrictions are indicated. There may be charges if there is damage or excessive cleaning required. It is advisable to bring blankets for heavy or shedding dogs, and they must be leashed or crated. This policy applies to the following companies. Reservations for rental cars usually don't have to be paid until you pick up the car. It is advisable that you check with the company about your reservation and pet(s) before you pick up the car.

Avis 
United States: 1-800-675-1487
800-352-7900; Customer Service 
Canada: 1-800-879-2847
Outside US 800-331-1212

Alamo 
877-222-9075

Budget 
U.S. 800-527-0700 
outside the U.S. 800-472-3325 

Hertz
US: 800-654-3131 
International: 800-654-3001

Enterprise 
United States, Canada, UK, Ireland, and Germany only 
US: 800-261-7331 

National 
877-222-9058

The following car rental companies have a more restrictive policy and should be avoided by people with pets unless they are comfortable with their pets remaining in a kennel in the rental car. Here is their pet policy:

Dogs of all sizes are allowed for no additional fee and they must remain kenneled while in the vehicle. There may be charges if there is damage or excessive cleaning required.

Thrifty
800-THRIFTY (847-4389) within the United States and Canada 

Dollar Rent-A-Car
800-800-3665