The videocassette is elbowing its way up to the souvenir counter. It's taking its place among the T-shirts, the postcards and the plastic models of the Washington Monument.

Forget the little selection of 35 mm slides. Now you can take Washington and its points of interest back home in video form.

You say you liked the historic film presentation on John Brown at the Harpers Ferry visitors center? Fine. Just pick it up in video before you drive out of town.

You enjoyed "To Fly!" at the Smithsonian's Air and Space Museum? Terrific. Take a copy home and show it to your friends.

In an age when about one-third of American homes have VCRs, videocassettes are becoming one of the slickest -- and priciest -- of souvenirs.

They're catching on at the Smithsonian Institution, and the National Park Service is into video -- meaning that just about everywhere you tour in the Washington area there's a cassette for sale.

Guest Services, Inc., the organization that provides concessions to the National Park Service, is especially pleased with "Washington, D.C., An American Monument," a half-hour video highlighting the tourists' view of the Nation's Capital. It's $29.95 and can be found at most of the places it features. "We just got it five months ago," said Ida Khater of Guest Services. "It's going well." And while video prices, typically in the $30 range, are not cheap, Khater noted that they generally ran to $40 a year or two ago.

Meanwhile, in West Virginia, Heather Walters, film librarian for the Harpers Ferry Historical Association, is busy developing film selections for the national parks. Typically, she said, there will arise a demand for one of the 16 mm highlight films shown at various park sites. The society catalogs, masters and markets the films in video form.

"We started last year," she said, "and found there was a big market. It's going even better this year."

The society has put out a catalog of 29 titles; about 13 selections are available at Harpers. Best-sellers there include "To Do Battle in the Land," a 27-minute video of the film of the John Brown story shown each hour at the Harpers Ferry visitors center. "Antietam Visit" runs 28 minutes; both cost $29.95. And former astronaut Wally Schirra narrates "Our Treasured Lands," a rundown on the national parks.

The Smithsonian offers a wide range of videos, from classic films -- "The Gold Rush" with Charlie Chaplin, Hitchcock's "The 39 Steps," "The Flying Deuces" with Laurel and Hardy -- to Chinese opera.

But of course the Smithsonian's big winner is "To Fly!" It's the video version of what is surely one of the longest running and most popular movies in history, a marvelous historical tribute to air travel.

The Smithsonian museum shop and mail order selections include videos on the history of the Civil War, World War II and the like, admittedly marginal sellers. They are augmented by the heavy sales of "To Fly!" seemingly forever the Smithsonian's hit parade leader.

Historical tapes "tend to be more expensive and sell to the person with a special interest," said Meredith Rinaldi, graphics buyer for the museum shops. Such tapes may move at the rate of only one a week, while "To Fly!" might sell 50 to 100 copies in the same period, depending on the season, she said.

"It's here to stay," said Rinaldi of the video souvenir. "We plan on keeping this category and will expand it . . . Not always do slow sales mean a tape will be discontinued" if it fills a need of Smithsonian patrons. "We hope that 'To Fly!' will offset the cost of carrying the others."

"To Fly!" can be purchased (in Beta and VHS) at the National Air and Space Museum shop for $29.95 or through mail order (include $1.85 for postage and handling).

In addition to becoming a souvenir, the cassette is also being used as a lure to get people to visit Washington or its neighboring states.

The departments of tourism in Washington, Maryland and Virginia and travel agencies and organizations are constantly on the lookout for new ways to promote the area. What they are discovering is that video is the way to go.

Each of the area tourism departments has a travel video designed to show people what's here. "We converted to the video format a year ago," said Tom Murphy of the D.C. Convention Center. "Video is requested now more than the 16 millimeter. Our film is primarily aimed at people who are selling travel. We loan copies to travel agents and they make copies."

A new approach has been developed by Cityscape Productions, which has made travel videos of Chicago, Nashville and Washington that are available to the consumer. The videos are designed to be "video postcards," not a travelogue with helpful hints, according to Cityscape president Scott MacKillop. "We try to make them as entertaining as possible with a musical soundtrack, visuals, but no narrator. They're designed to leave you with an impression about a city," said MacKillop. "It is sold to people who are visiting D.C. as tourists and people who are interested in doing their traveling from their living room."

MacKillop is especially pleased with the part of the video on the Vietnam memorial. "It is a very moving piece," he said. "I've seen tears well up in people's eyes. I'm very proud of it."

The 30-minute tape, available in Beta and VHS, costs $19.95 and is on sale at the Smithsonian museum, the National Zoo, the monument gift shops and through a mail order catalog. It's also shown for free at the Visitors' Center downtown.

Maryland's 10-minute promotional video has become "indispensable" to Charles Lehmann, assistant director of the state's department of tourism. "I've seen it pick up in the last year. I'd say the requests for it have tripled," said Lehmann. "I get very few requests for the 16 millimeter, now that everyone has VHS. It's really an inexpensive way to promote Maryland. I can send a $5 tape for $1.50 postage."

Lehmann does not send the tape to private individuals, but he feels it's just "a matter of time before they the tapes find their way into the video stores."

Besides the overall state video, Maryland has a one-hour cassette that combines videos from several counties including Frederick, Washington and Harford. St. Mary's County has two videos, one devoted to fishing, and Garrett County has a video on white-water rafting. Those are a little more generic because the counties are not able to spend as much for production, according to Lehmann.

He said the state also has plans for producing a longer 25-minute film on the state that should be ready by the end of 1987.

Virginia has a 10-minute and a 26-minute travel video promoted by the state's division of tourism titled, "And You May Find . . . Virginia." The film is geared for cable television, travel agents and tourism guides. At present the department is not equipped to deal with consumer response, according to Sue Brinkerhoff of the Virginia Division of Tourism. But she said the agency would probably allow individuals to borrow the film and copy it.

The state also has a video called "In The Middle of Everything" on the Virginia peninsula and also videos on Gunston Hall and Stratford Hall that are available on a free loan basis to television stations and adult group organizations.