Virginia officials are considering four potential sites for a major league baseball stadium in Northern Virginia, including a Rosslyn location on the banks of the Potomac River overlooking the Washington monuments and another at the edge of the suburbs near Dulles International Airport.

Sources close to the Virginia Baseball Stadium Authority and in Arlington say two other possible locations are in the heart of that county's Pentagon City area -- on land occupied by a Costco warehouse store or on nearby property close to the popular Pentagon City shopping mall.

Virginia is competing with the District and Portland, Ore., to become home to the Montreal Expos, a struggling franchise that Major League Baseball officials say they will relocate, perhaps as early as next year.

District officials are openly discussing their possible sites for a new stadium, which include an area by a new Metro stop on New York Avenue NE and a site near Union Station. Another site is on the Anacostia River waterfront, near South Capitol Street, and a fourth is just north of Robert F. Kennedy Memorial Stadium. Mayor Anthony A. Williams (D) said last month that he opposes a fifth proposed location, on the eastern side of Mount Vernon Square.

Virginia officials have refused to go public with their locations, saying they first want to finish noise studies, traffic analyses and environmental impact statements. The Virginia sites were first reported in Marc Fisher's column in early Sunday editions of The Washington Post.

"It just seems sort of unnecessary for everybody to start people looking at pros and cons and commenting one way or another," said Fairfax County Supervisor Michael R. Frey (R-Sully), the chairman of the authority. "That's unnecessarily raising fears."

Indeed, the locations may spark an intense reaction from neighborhood and civic organizations, as happened seven years ago when opposition to several proposed sites swelled even though Major League Baseball had not decided whether to locate a team in Northern Virginia.

Arlington officials cautioned that the three options in their county, while all close to Metro stations, would likely put additional pressure on the county's roads and parking infrastructure.

"There would be a lot of hurdles" for each of the sites, said County Board Chairman Paul Ferguson (D).

Board member Chris Zimmerman (D), who has long opposed building a stadium on any site in the county of 189,000, said: "I think that once people realize the economics of any plan, they'll realize how bad it is for Arlington."

The Dulles Airport area, while potentially more spacious than some close-in sites, is not near a Metro stop, and a stadium proposal might generate fears among homeowners of traffic jams on already clogged roads.

The Rosslyn site is a complex of four apartment buildings that sits next to the former USA Today and Gannett towers. The site is near a Metro stop, has stunning views of Washington and would allow quick access to Interstate 395 and the District. The 1,400 apartments are owned by hundreds of investors in a cooperative, making acquisition of the site a challenge.

The two Pentagon City sites are next to a Metro stop and near the District. The Costco, which is one of the chain's most successful stores, would have to be torn down to make way for the stadium. The other property, partly occupied by warehouses and partly vacant, is owned by the descendants of developer Morris Cafritz.

"I think if you are going to have a stadium, it should be the one by the river," said Del. Vincent F. Callahan Jr. (R-Fairfax), chairman of the Appropriations Committee in the Virginia House and a baseball booster. "You have a D.C. presence overlooking the nation's capital, you have transportation."

The three Arlington sites are the same ones mentioned by county officials last summer as potential locations for a new conference center. The county, which does not have a place to host large-scale conventions and shows, recently won approval from the General Assembly to use hotel taxes to finance a facility.

In a statement issued last week, stadium authority Executive Director Gabe Paul Jr. said a consultant will study the possibility that a single facility could include a baseball stadium and a convention center in Arlington.

"One co-development concept that has been suggested is for combining the ballpark with a mid-sized conference or exposition center," Paul said.

Several community activists said Metro's Orange Line, which serves Rosslyn, is already at capacity on weekday nights.

"My primary concern is that the infrastructure isn't there to support" a stadium, said Mike W. Clancy, a county activist who is running as a Republican in next month's special election to fill the County Board seat of Charles Monroe (D), who died last month. "Any of the plans would not only severely impact surrounding neighborhoods with increased traffic but where would people park?"