You might think that a fake colonial farm staffed by National Park Service personnel wearing cute 18th-century costumes is a frivolity left over from Bicentennial days. You might think so, until you listen to some of the staff's stories. Turkey Run Farm, which recreates the lifestyle, of a poor Northern Virginia farm family of the 1770s, is more than another bit of patriotic fluff; it's a public service. Many of its young visitors are blissfully ignorant of Life Before Techology.

One kid, says Jim Runkles, who plays the colonial father, searched the log cabin and demanded to know where their television was hidden. He knew they didn't have electricity back then but figured they must have had a wooden set tucked away somewhere.

Another little boy who watched Runkles decapitate a chicken wouldn't believe it was a chicken until he dipped it in hot water and pulled the feathers out."Then he recognized it because it looked like the chickens he was used to seeing - the ones on the supermarket shelves."

Turkey Rum Farm, located about a mile from the high-speed traffic of George Washington Parkway in McLean, does what it can to correct these misconceptions. The agricultural and household activities that visitors see depend on the season, weather and time of day, but all are typical of a poor farm family two centuries ago - "when small farms were dispersed throughout the rustic countryside, and the majority of Americans were engaged in activities of an agricultural nature," according to the old-timey type in the farm's brochure.

Working at Turkey Run isn't exactly a glamor job, as was evident one recent afternoon at pig-feeding time. Rosemary Taft, the young woman who plays either the colonial mother or the widowed sister (depending on the day of the week), slopped half a bucketful of dishwatery-looking liquid into a wooden trough as four small visitors hung over the fence in anticipation.

Finally Timothy and Persimmon, the colonial pigs, came snurfing up, pushing each other out of the way as they inhaled the stuff through their nostrils. "Ewwww," said the kids. "They don't have very many manners." "Pigs don't need manners to be accepted by other pigs," Taft told them. "Is that all they get to eat?" another asked. "It may look awful to you," said Taft of the beet-juice mixture, "but it taste great to them."

These are the kind of Bicentennial Minutes that stick. In the short walk from the pigpen back to the cabin, the kids learn about colonial meals, marriage customs (girls were considered old maids if they weren't married off by age 14 or so) and clothing (the long skirts are indeed as hot as they look, but they have their advantages; the bugs can only get at your ankles.)

Back at the cabin, the "family" - today it's a mother, father and two kids - goes about the afternoon chores while the four visitors tag along. It's hot and humid and the flies are ferocious. One of the 20th - century softies complains about the bugs, and Taft leans over the kitchen garden fence to pick some wormwood leaves to crush and rub on the skin. The colonial version of "Off!" doesn't seem to make much of an impression on the 20th-century bugs, but it is mildly refreshing. Very mildly.

Despite the heat, there's a fire in the fire-place - Taft is baking bread on the hearth. It smells great. "These old iron bake kettles make a much better loaf than any electric oven," she says. She makes two loaves every other day, using only ingredients a poor colonial family would have had access to. Everything's right from the cow, the chicken or the earth: whole-wheat flour, butter, eggs and sour milk.

The farm staff likes to involve visitors in their colonial lifestyle, letting them fetch wood for the fire, help feed the animals or dry the dishes. Many kids get so involved they'll stay for hours, Taft says. "The few who say 'Is this all there is?' are a minority."

Today's visitors are full of questions as they trail around after the various family members. "Do you really eat only what the family would've eaten?" (Yes.) "What do you drink?" (Water, cider from their apples or milk from their cow.) "What holds these cabin logs together?" (Clay mixed with a little lime.) "How did everybody fit in here, anyway?" (Most of the time they were outside, doing chores.) "Even in the winter?" (Sure, there's always fences to mend, animals to feed . . .)

Adults want to know if living without modern conveniences makes them appreciate 20th-century life more. Taft says it has the opposite effect, making them appreciate the life we're losing - taking vegetables out of the ground, working with real materials, not having any trash. "There's no trash here," she marvels. "Nothing is packaged."

Living 18th-century style is not romantic, she concludes, especially in the summer. Working in the heat and humidity can be draining, and you never do get used to the flies.

"But we 20th-century people are learning a lot about survival, working here." FARM FACTS

Turkey Run Farm is open free of charge Wednesday through Sunday, 10 to 4:30, through November (weekends only from December to March). Parking is limited to 30 spaces.

There are also evening concerts the second Thursday of each month. Volunteers play music on ancient instruments and everyone joins in the dancing "the way they did in the old days, when the spirit moved them."

To get to Turkey Run, take the Capital Beltway to Route 193 (Old Georgetown Pike); follow 193 east toward Langley for 2 1/2 miles, then go left into the farm. Or take George Washington Parkway to Route 123, go right into the farm. For details, call 557-1356. Large groups should call 557-8991 in advance. OTHER FARMS

Three other model farms in the area seek to evoke the past:

1770s: NATIONAL COLONIAL FARM

Bryan Point Road

Accokeek, Maryland

301/283-2113

DIRECTIONS: Take Capital Beltway Exit 37. Go south on Indian Head Highway (Maryland Route 210) for 10 miles to Bryan Point Road; turn right and follow the road to the Potomac River, about four miles. Hours: 10 to 5, Tuesday through Sunday, June through Labor Day, weekends after that. Admission: $1 for adults, 50 cents for children. The best thing about this 140-acre farm, jointly run by the private, nonprofit Accokeek Foundation and the National Park Service, is its location. It's beautifully situated on the banks of the Potomac, with a gorgeous view of Mount Vernon just across the river.

The farm is a curious blend of old and new. It's supposed to depict "the agricultural methods, crops and livestock of a modest. Tidewater farm on the eve of the American Revolution," but there's been no effort to maintain a strictly colonial facade.So volunter farmers use power tools; ladies in colonial dresses wear sneakers and sunglasses; plastic green hoses with shiny silver nozzles lie around the grounds. You never quite forget you're in the 20th century.

But you can't beat it for peace, quiet and beauty. The real winner in all this has to be the 19-year-old Park Service employee who maintains the place. He and his family live, rent-free, in what must be one of the prime chunks of real estate in the Washington area; a waterfront board-and-batten farmhouse with a view of Mount Vernon from the front door.

Visitors can see farm crops (grown by hand without herbicides or insecticides), a beautifully maintained herb garden, lots of farm animals and a Woodland Food Trail with markers identifying plants, trees and shrubs used by colonists. The farm is also the site of genetic research on the major crops of the colonial period and produces seed stocks for itself and other agricultural-historical museums.

It's also a great place to buy fresh vegetables in season. Also for sale are dried herbs, herb vinegar, purple martin birdhouses made out of dried gourds, guidebooks, and a great-smelling colonial herb tea made from, among other things, lemon balm, woolly apple mint, spearmint, strawberry leaves, comfrey, red clover blossoms, lavendar blooms, rose petals and camomile.

1920s TO'40s: FRYING PAN FARM

West Ox Road

Hendon

703/437-9101

DIRECTIONS: From the Capital Beltway, take U.S. 50 west past Fairfax Circle (it becomes Lee-Jackson Highway); turn right on West Ox Road and follow it to the park on the right. Free admission.

This is supposed to be a 1920s-era subsistence farm, but don't come expecting to find the Waltons. The Fairfax County-funded project is short on staff and the visitor is likely to be greeted by only the animals: Jack and Jill the Clydesdales; Sugar the pony; Jenny the mule; Big Red, Snow White and Poker the pigs, and assorted geese, ducks, peacocks, turkeys, rabbits, goats, sheep and chickens. If you come between 4 and 6 in the afternoon, you might see some of the farm chores being done. Or you can call ahead and try to arrange a tour. But, said one volunteer, "Usually kids just kind of walk around and say, 'Hey, Mommy, look at that,' and if they're lucky a volunteer will come around and answer questions."

There are lots of interesting-looking antique farm machines around, for the history-minded. And they sell fresh eggs (75 cents a dozen.)

1900s OXON HILL FARM

6411 Oxon Hill Rd, Oxon Hill

839-1177

DIRECTIONS: Take the Capital Beltway to Exit 87 South and follow the signs: turn right off the ramp onto Oxon Hill Road, and make another quick right into the farm. Summer hours: Mondays through Fridays 8 to 5, weekends 8:30 to 8:30. Free admission.

Oxon Hill Farm, a National Park Service program in a convenient location right off the Beltway, is most like a real farm of all the ones listed here. There are barns, stables, pastures, crops, scarecrows and lots of animals. This is the place to go to introduce your kids to a variety of real, live farm criters. Judging from some of the comments, there are lots of city kids around who still find that prospect exciting. "I'm gonna pet this horse!" yelled one 10-year-old boy to his friend. "You dare me to?"

Besides the horses, there are some absolutely beautiful silky-haired goats; an equally impressive-looking herd of sheep (who, by the way, really do say "baaaah"); a family of spotted pigs wallowing in the mud like hippopotamuses - one was so submerged he looked like a big brown rock; and the usual assortment of cows, donkeys, chickens and ducks. They also have a nature trail, and farm activities such as corn harvesting, wheat threshing, cider pressing, spinning and sheep-shearing in season.