These days, it seems a celebrity's walk down the red carpet is simply not complete without a twitchy toy dog atremble on her arm.

The biggest player in this "pup tart" phenomenon is Paris Hilton, whose Chihuahua, Tinkerbell, rode shotgun on her mistress's reality show, "The Simple Life," piddling on the threadbare carpets of modest American host families the nation over.

Not to be outdone, pop star Britney Spears has a trio of pups whose constant housebreaking gaffes reportedly have left some Jackson Pollock-like effects on the white carpets of her Malibu mansion: a Maltese named Lacy Loo, and two Chihuahuas, Bit Bit and Lucky.

Even P. Diddy has a Maltese called Sophie, whose crib is a Louis Vuitton bag.

When will it all end?

Not soon enough for critics of these high-profile "pocket pets," who object to seeing a living creature treated like a Birkin bag with an Iams habit.

"These are not bracelets, wristwatches or pocketbooks -- they are sentient beings," says Darlene Arden, author of "The Irrepressible Toy Dog." "In every picture you see of these twenty-something celebrities, they're carrying around a dog, usually a Chihuahua or sometimes a Maltese, like an accessory. But a dog is for life -- not for an evening out."

Perhaps not surprisingly, the fallout has already begun. Lucky, the Chihuahua owned by a now-pregnant Spears, recently got the boot to an assistant's house after attacking the ankles of husband Kevin Federline. (Spears reportedly visits not-so-Lucky on the sly.)

"These dogs have very special needs, and they are not for everyone," Arden says. "In some ways, it's like having a premature baby."

In cool weather, toy dogs need to wear sweaters -- not as a fashion statement but to retain body heat. And they are delicate, Arden says. "If you step on one, you can kill it; if you trip on one, you can paralyze it; and if you roll over on one during the night, you can smother it."

That vulnerability goes double in the outside world, where a few unattended minutes in the back yard can court disaster on any number of fronts: loose dogs and cats scaling the fence, a hawk swooping down from overhead, or that most dangerous species of all, a human, quietly opening the gate and walking off with your diminutive pile of fur.

Bred solely for companionship, toy breeds need contact with the humans in their lives. Absences or stays at a boarding kennel will not be taken well, even if it is Cannes week.

Owners also need to be tuned into a toy dog's Lilliputian perspective.

"If you only come up to people's ankles, what does the world look like? It's huge, including a human hand coming down on you," Arden says. Slow, smooth movements and scratches under the chin, not over the head, are in order.

As for Lucky, his obstreperousness should not come as a surprise to anyone who has read the Chihuahua breed standard, which notes its "terrier-like qualities of temperament." "Unless you have one that's been bred for good temperament," Arden warns, "they can bite first and ask questions later."

Finding a vet who is comfortable working on such a small package can also be a problem. Arden says she often recommends vets who specialize in cats "because they know how to moderate their touch to small animals."

Like their larger counterparts, toy dogs are susceptible to various health problems, from liver shunts in Yorkies to kidney problems in Shih Tzus to a blinding disease called progressive retinal atrophy in papillons. Luxating patellas, or knees that pop out of their sockets, are persistent problems across most of these small breeds, as are collapsing tracheas.

"Toy dogs can go down really fast, and they don't have the reserves that a larger dog does," Arden adds. Puppies and so-called "teacups" are especially susceptible and can easily become hypoglycemic if not fed every few hours and kept constantly warm.

As for Paris Hilton's beloved Tink, the wily Chihuahua may be spared the vagaries of fashion, if only because she has proved herself a cash cow, with a book ("The Tinkerbell Hilton Diaries: My Life Tailing Paris Hilton") and a deal with Gund, which will manufacture stuffed animals in her likeness. Plush, indeed.