Thanks to Little Bit, the horse with a big appetite, and Jim, the husband with a keen sense of a family budget, Labor Day required a day of labor from Marcella Birch.

"Jim said, 'If you want that horse, you work," said Marcella, explaining why she was on the job at Lane One on the western end of the Chesapeake Bay Bridge where she was welcoming the holiday hordes back to reality and collecting $1.25 per "class one" vehicle.

To hundreds of thousands of other workers in the metropolitan area, including Marcella's husband Jim, who had planted himself at a fishing hole in the Chester River, any labor more strenuous than hoisting a piece of chicken was positively abhorrent.

Judging from yesterday's observance of the day that unofficially marks the end of summer's liberal regime, the metropolitan area still finds Labor Day traditions as comfortable as a chaise longue under a shady tree.

As the temperature hit 94 degrees on an afternoon that was thoroughly in keeping with this summer, the hottest in the nation in 36 years, the streets of the capital looked as if the populace had successfully carried out a civil defense evacuation exercise. The doormen at the Madison Hotel tugged off their white gloves in hopes of cooling down. On Capitol Hill it was hard to turn up a lobbyist, and those inimitable street philosophers of wine had plenty of room to discourse and nap in parks that normally swarm with young professionals.

In order to find the city, you had to follow the smell of barbecue smoke to Hains Point, or sniff after the aroma of Bain du Soleil and Coppertone that lingered in the air from the District to Ocean City. More than 150,000 people flocked to Ocean City for a last fling with the Atlantic Ocean, police said. Looking askance at the holiday mood, police arrested scores of revelers on drunken driving charges.

The roads abounded with signs of the changing season as boats under tow headed home for months of dry dock. On the shoulder of Rte. 50, in Prince George's County, four people sat in a trailered cabin cruiser behind a truck becalmed by engine trouble.

At Sandy Point State Park, in the shadow of the Bay Bridge, there was something joyless in the way a Korean father squirted his charcoal with lighting fuel and something elegiacal in the way his family sat quietly under a pine tree. Perhaps they knew it was the last barbecue. Perhaps the summer had dragged on too long. Perhaps it was just too hot.

"You think this is hot?" asked Mary McClain, squinting at the water off Sandy Point. McClain celebrates today 10 years as a shirt finisher at Esquire Cleaners in Springfield, where the temperature must be "at least 200 degrees." Since June she has been out with a broken leg. It just snapped, she said, because her bones had gotten tired from her standing up all the time. Despite crutches, she was determined to hobble down to the sea with her nephews Anthony and Michael Underwood.

"It'll take me a month to get down to the water," she said, "and another month to get back. It's a full day's work just standing around."

In Baltimore, the lack of jobs spurred about 1,000 people to observe the Labor Day holiday with a protest march organized by the Maryland-D.C. Council of the AFL-CIO's nationwide demonstration against President Reagan's economic policies. Council president Tom Bradley used the forum to call upon the unemployed to band together "in a unified effort to defeat Reagan in 1984."

In Washington, organized commemorations of Labor Day included a mass at the Shrine of the Sacred Heart in the District in honor of Dr. Martin Luther King Jr. Lawrence J. Cain, an official of the International Union of Operating Engineers, said the service also was intended "to revive the old spirit of celebrating labor. The service would "commemorate the birth of Polish Solidarity and its continuing struggle," he said.

At an Irish Music and Dance festival at Western Plaza Park, 200 people gathered to watch Maura McMahon of Rockville pirouette her way through a complicated dance routine and to otherwise join in paying a "compliment to the Irish workers who have helped to develop this city," as one of the festival's spokesmen put it.

Traffic on the Bay Bridge was lighter than expected yesterday afternoon, according to bridge police. But that didn't mean that drivers pulled up to Marcella Birch's mercifully air-conditioned toll booth reciting love poetry.

"I'm not a dog," said Birch, a 23-year-old woman wearing a Maryland state-issued toll-taker's blouse, canvas shoes and a pair of pants her husband secured from the Baltimore City police department. "I'm not a machine. I'm a human being."

There were a few grudgingly human "hellos" and "thank yous." But most of the returning travelers, braking to a halt at the rate of one every five seconds and looking dreadfully put out, stuffed a dollar and a quarter into the toll-taker's hand and mashed the gas pedal, leaving her to choke in a cloud of carbon monoxide. She gets bruises on her forearms when motorists slam the money in her palm and bang her arm against the stainless steel dutch doors, she said. Several motorists snapped at her to hurry up and one man barked, "I'm dying!"

Sometimes drivers run the toll, she said, and sometimes they expose themselves. In Birch's experience more expose themselves than run the toll, which seems significant in some indefinable way.

The cars pausing at the toll plaza--Volvos, Winnebagos, Fords and Chevrolets--contained a seasonal miscellany of items: paper towels, light bulbs, oranges, a loaf of cracked wheat bread, a case of Soave Bolla, rowboats, nets, typewriters, flowers, tennis rackets, crab pots, squads of children and one golden retriever.

Birch, finishing up her 7 a.m. to 3 p.m. Labor Day shift, collected $2,224.20 for the state. She picked up her radio, her soda-cooling sleeve of styrofoam and her radio, and headed home to Centreville to put her feet up for what was left of the day.