The annual Washington benefit pet show came of age on Saturday, but for a few cloudy early-morning hours it looked as if it might not come off at all.

"It was like D-day this morning," said ringmaster Art Buchwald. "Everybody was calling everybody from about 8 o'clock on, asking, 'Do we go?" 'Don't we go?' Now I know how Eisenhower must have felt."

But, in the end the weather cooperated and the invasion was on at Hickory Hill for the 21st anniversary of the event, held this year, as in the past four, to benefit Runaway House, a runaway counseling and foster care program for young people.

Troops of nervous cats, high-strung dogs, ruffled birds and placid fish bellied, squirmed, flapped and sloshed onto the seven acres of sloping, slippery lawn. The four-footed invaders had their bipeds in tow and they, in turn, struggled, trudged, slithered and were dragged into the fray.

Firmly entrenched representatives from nine embassies sold native delicacies from tables on the terrace, while a combined force from eight Washington restaurants provided sweets for the famished forces at Ethel Kennedy's McLean, Va., estate.

For those who were superstitious about pet-show strategy, there were para-psychological consultants: a palm reader (no one was doing paws), and a mentalist, Neil Caesar, who gave demonstrations of his psychic powers. R and R was provided in the slide-for-life, celebrity tennis, an obstacle course, soccer with the Diplomats, pony rides and a dunking machine. For the shell-shocked, there was a pet psychiatrist.

The outcome of the skirmish was a draw - everybody came away with a ribbon.

Leslie Talmadge, 10 ("no relation," she said hurriedly), had entered a neighbor's cat in the "largest" and "best all-around" categories, but came away with a second in "best behaved".

"I don't think we should have gotten that," said the redhead as she struggled with the aptly named "Chicken' ("I won't tell you her middle name - it's not good . . . it begins with an 's'"). The gray and white American shorthair she had on her leash was clutching the ground, preparing to bolt, sheer terror gleaming from her eyes.

Helen Hooper was recovering from a hollow victory as she rested on the lawn between the cages of a parrot and a cockatoo, owned by Henner Wachs. No, they hadn't done badly, she said, "considering there was no competion and we won first prize."

Still, there were some winners for whom the victory was swift and sweet.

One even made a gracious and eloquent acceptance speech when she won the most attractively dressed prize. Petunia, a five-week-old pig, dressed in pink, squealed and squawked into the microphone as the ringmaster translated.

"You say this is the first time you have won a contest? And you want to thank your mother and your father and your sisters and your brothers? You especially want to thank Secretary Califano for all that he has done for you and others like you?"

Other winners:

Most celebrated celebrity. Gary Coleman from television's "Diff'rent Strokes," who was surrounded by masses of children and adults wherever he went, mostly held high on his father's shoulders - one suspects not only to get a better view of what was going on.

Most unusual pet.A tie between a jar of recently hatched praying mantises and two ferrets.

Least missed of last year's guests. Suzie, the 6,000-pound Indian elephant, who broke away from her handlers and cut a swath through last year's crowd not unlike the trail Sherman hacked through Georgia, as terrified guests, including Amy Carter, scrambled out of her way. Amy, who had been hoisted out of Suzie's path by a Secret Serviceman - who broke his finger in the line of duty - was back, however. Seemingly undaunted by last year's trauma (or by any political tensions between her daddy and Hickory Hill's "Uncle Teddy"), she made her way around the grounds, nervous looking Secret Serviceman in tow. It wasn't known if last year's hero was among her entourage.

Most dapper celebrity. Columnist Rowland Evans who was dressed in powder-blue checked seersucker pants, red-checked seersucker blazer and straw boater. "I wear this costume only once a year, and I have worn it here for over 12 years," he proclaimed.

Most courageous celebrity. Gary Coleman, who took a demonstration ride on the slide-for-life, a trapeze-like contraption, in the lap of one of his entourage and was promptly encouraged to do it again solo "for your fans." And did.

Most exuberant. Ethel Kennedy, who ran around the obstacle course, cheering her children. ("Will you get a picture of Bobby for me?" she asked a nearby photographer, then was off, running beside him with "Bobby, run . . . hurry!")

Most softhearted. Joseph Califano, who could not decide on the first prize for the greatest hard-luck animal in the show, so split it between a dog from Hungary and another that was both blind and deaf. (Second prize went to a dog that had voluntarily committed himself to the pound before he was adopted by his present owners.)

Dirtiest. A tie between Gary Coleman and Amy Carter, both of whom ran the wet muddy obstacle course.

Best dressed: Celebrity division winners were the cast of the "Wiz." In the pet category, winners were three siblings and their large dog, all dressed as Dips.

Oldest. The Kennedy turtle which has shown up every year the show has been held at Hickory Hill.

Most honest. Art Buckwald, who said: "I see the same old turtle is back. It's not as unusual as it was 11 years ago, but every year it wins a blue ribbon. That's why Mrs. Kennedy lets us use her house for the pet show."

Best imposter. The scottie in the costume class, dressed in green because, as his owner explained, "He's an Irish dog."

Most ubiquitous. Gary Coleman who, after he was mopped up from his tumble into one of the puddles on the obstacle course, entered the ring to judge the fin class. The goldfish was pronounced "kinda nice and dirty . . . pretty good." The frog was proclaimed, "A good frog . . . I like it."

There was a ribbon for every entrant, so theoretically everyone went away satisfied. But for the fiercely competitive, every contest means war, as ringmaster Buchwald pointed out.

"A thing like this leaves a scar on you for life," he said, eyes roving over the melee. "I've had 18- and 19-year-olds come up to me and say, 'I remember that you only gave me a third when I was 8.'" CAPTION: Picture, Samantha Epstein, 4, and her dog, Snuffy; by James A. Parcell