One of the largest tracts of open land in eastern Loudoun County is under contract to two major residential development companies, officials said, assuring that hundreds of new homes will take shape on pastures and wheat fields north of Dulles International Airport.

An 865-acre parcel--part of a 1,500-acre farm owned by the Claude Moore Charitable Foundation--is being purchased by Pennsylvania-based Toll Brothers Inc., and an adjoining 79 acres of the farm is under contract to Richmond American Homes of Denver. Together, the 944 acres are going for about $50 million, according to sources close to the sale. The trust expects to put the money toward the educational programs that are its main focus, officials said.

Cory DeSpain, regional vice president for Toll Brothers, said company officials haven't decided how many homes they will build, although existing zoning could allow as many as 1,400.

"We're so new into this purchase we haven't resolved all our plans," DeSpain said.

Richmond American Homes plans to start building about 70 homes on its tract next year, according to Bill Ostrander, a company vice president.

The deal comes as officials in Loudoun, the nation's third-fastest growing county, are becoming increasingly concerned about covering the costs of new schools and other services needed for its growing population. Newcomers are arriving in the county at the rate of about 1,000 per month.

"We're always concerned about the pace of growth in Loudoun County," said Sterling Supervisor Scott K. York (R).

Toll Brothers has become a major player in Loudoun development. The company has recently purchased 2,950 empty lots in South Riding, south of Dulles. Toll Brothers could also construct up to 820 apartments and 4.2 million square feet of office, retail and industrial space on additional land it owns in the area.

On a farm west of Leesburg, the company also has plans for 130 houses, and it won approval last year to build 1,933 houses and a golf course west of Ashburn.

The Claude Moore property is bordered roughly by Route 772 to the north, the Dulles Greenway to the east, Route 659 to the west and Broad Run Creek to the south, according to foundation lawyer Randy A. Sutliff.

Because of its size and location near the thriving high-tech Dulles corridor, the tract "is a prime piece of property," said Julie Pastor, Loudoun's director of planning.

County officials have envisioned a mix of homes and commercial space for the land, Pastor said. A stadium or conference center served by a rail connection to a planned spur to Dulles could also be part of the mix, according to county plans.

But development there could have a down side. Some parts of the property lie within a mile of the "noise zone" around Dulles, which is the region's fastest-growing airport.

"All potential buyers must be notified that you're likely to see and hear planes, being that close to the airport," said Neal Phillips, who oversees noise control for the airport.

Sutliff said trustees for the estate of Claude Moore, an eccentric millionaire and conservationist, have been seeking proposals to sell the land since March. He said he expects the deal will be finalized early next year.

"The reason we're selling now is that the trustees have decided they're getting a good price for it," Sutliff said. "The purpose of the foundation is for educational uses. The land doesn't help with education, so they thought they should get it turned into money and get on with their mission."

The proceeds from the sales of the Moore property will go toward educational and literacy causes, Sutliff said.

About 600 acres of the foundation's land just north of the site being purchased by Toll Brothers did not go under contract because the trustees didn't receive an offer for it that they thought was acceptable, Sutliff said.