The website is http://tattoo-a-pet.com - they are the national registry, and have the database of all the pets and their ID numbers. They also give the owners a tag to put on the pet's collar with their 800-number on it, but even if the pet isn't wearing a collar, a vet or groomer who sees the number can always call.
It's only $35, and seems well worth it.
When someone asks why I don't have any tattoos, I tell them that with my luck, I'd be allergic to the dyes, and it's just not worth the risk. Anyone who knows me well knows I have all sorts of allergies, including a bunch of food allergies, so I'm not taking unnecessary chances!
Bookmarks