The Hickory Hill Pet Show, which was held Saturday on Ethel Kennedy's extensive lawn, pool and mud puddles, has been going on for 19 years now.

An animal who won a prize for being Smallest in its youth ought to be ready, if it has made any use of the intervening years, to quality for Most Colorful.

The Kennedy children, who used to toddle about with big things on leases, collecting the Most Awwwwww's, are now great bulking people who beat the paying guests on the obstacle course.

This year, the Most Watched Celebrity was fighter Muhammad Ali, who went on the Slide for Life and volunteered to go up in the Hot Air Balloon Ride; but didn't have to because there was some problem with it.

He was also supposed to be judging pets, as an assistant to ringmaster Art Buchwald. But Ali refused to decide between two cats, either because he couldn't make up his mind or because he didn't want to risk offending the loser, so he was assigned to pin on ribbons.

He also volunteered that his favorite pets are horses. There are no horses at the Hickory Hill Pet Show.

However, there were: a three-legged cat, a basset hound dressed as Buchwald and on its way to the Sans Souci, and a rat and boa constrictor who had lived in the same cage for three days with both surviving, who got a peace award.

And there were lots of Redskins, conducting the obstacle course and competing on it with U.S. Marines; various legislators and other agile types doing "Celebrity Tennis" and the Diplomats Soccer team, playing with whoever wanted to play.

There was Ethel Kennedy, wearing a T-shirt on which was spelled out in calico, "Better the Dogs Should Go to the Country," and there were lots of other T-shirts, ranging all the way from Cardin's C's to Anne Klein's lions (none of which attempted to enter the ring, however). One pet had Gucci leash.

There were goodies to eat from the children's favorite snack shops, such as Jean-Pierre, the Big Cheese and Lion d'Or. And the pizzeria had the Ladies of the Italian Embassy as cooks and waitresses.

In front of the Kennedy house were not only guards, but a sig informing the public that "Trespassers Will Be Eaten."

This year's show, at $5 a ticket, was a benefit for Special Approaches in Juvenile Assistance, a youth services project which includes shelter and counseling for children from 11 to 17.