Edward M. Burdette was there to show off a quarter of a million dollars worth of purebred Holsteins; Warren Wallring came from Baltimore to peddle a $465 machine that he says eases back pain and improves memory at the same time; and Derek Smith, Mike Carretta and Scott Cardillo, three teen-agers from Gaithersburg, arrived, they said, to "check out the girls."

The Montgomery County Fair, a week-long festival with old-fashioned farming arts and new-fangled merchandising, opened yesterday at the Gaithersburg fairgrounds. Hundreds of people turned out in a 97-degree haze to nibble on a slice of an 8,000-pound wheel of Wisconsin cheddar, pet a farm animal for the first time and to gaze at preserves and baked goods produced by Montgomery's finest cooks.

"This is an urban fair that's still a showcase for agriculture," declared Roscoe Whipp, 65, president of the Montgomery County Agricultural Center Inc., which sponsors the annual summer rite. Whipp, a Frederick County native who was a Montgomery extension agent for 31 years, helped organize the first county fair in 1949.

"Development down county has taken away some of our best farm land," said Whipp, "but we still have 125,000 acres of prime tillable land left. Our job at the fair is to educate urban people about the county and its agriculture."

For Ed Burdette, a third generation farmer from Laytonsville, that means answering questions about the kind of milk his brown Jersey cow produces. "People ask me seriously whether the milk is chocolate or even the same color as the Holsteins'," said Burdette, 40, who oversees 300 dairy and breeding cattle on his family's 300-acre Flag Valley farm.

Burdette, whose cattle have repeatedly won top honors at state-wide livestock shows, said the fair is a time "to see how you and your cattle stand up to the next guy.

"The whole point is self-satisfaction," said Burdette as he stood near Thesus, a 1,200-pound black and white Holstein that won a state-wide bull calf competition last year. "And it's also a time for promotion: what you do at the fair can eventually turn into dollars." This year's cattle judging is scheduled for Wednesday.

Other events include a draft horse competition tonight, a popular tractor pulling contest on Wednesday and Thursday, a demolition derby on Friday and a controversial mud-wrestling contest on Saturday by a group of women from Chicago.

The decision by fair organizers to pay $2,000 for a performance by the Chicago Knockers raised eyebrows among the fair's more conservative members, officials acknowledged yesterday. But Whipp defended the event, saying, "The only thing dirty about it is gonna be the mud."

Warren Wallring said his small stall on the long fairway was a departure from the "malls and shopping centers where I usually work.

"But I was told that the Montgomery County Fair might be a good place to demonstrate the machine," said Wallring. His "Backswing" machine--a sort of seesaw for the human body--relaxes back muscles and increases blood circulation to the brain, improving memory and concentration, Wallring said. "I don't know how many I'll sell at this fair," said Wallring.

"But I got a lot of interest last week at the Howard County Fair. I even sold one."