Nearly 300 acres of historic, sloping farm land near Leesburg will soon belong to the Northern Virginia Park Authority as a result of what officials said yesterday is the largest donation of land ever made to the authority.

Mrs. James H. Symington is giving Temple Hall Farm, a 286-acre cattle and hog farm in Loudoun County where she has lived for 45 years, to the authority, requesting that the land permanently remain a working farm.

"In total acreage, this is close to three times larger than any donation we have received in the past," said Darryl Winslow, executive director of the park authority.

The donation will become official on Sept. 27 -- exactly 45 years since the Symingtons moved to Loudoun and began farming the land.

Symington took over operations of the farm in 1974 after the death of her husband, brother of former senator Stuart Symington (D-Mo.).

After learning the business, Symington said, she became as attached to the business as her husband had been.

"I just love it to pieces now," she said. "I thought if there is any way I can keep from selling it I want to avoid that, because then I have no control over how the land is used."

Symington has been distressed at the increasing development that Loudoun has experienced in recent years. "The county has just bombshelled," she said. "In the 45 years I've been here it has just completely changed."

Park authority officials emphasized the historical significance of their new property, whose value was not announced. The land was originally owned by Lord Fairfax, who sold the property in 1740 to Ann Thomson Mason, whose son George wrote Virginia's Declaration of Rights.

In 1825, James Monroe and President John Quincy Adams visited Temple Hall for the christening of two of Thomson Mason's daughters.

Symington, 69, will retain life tenancy of the two-story red brick farmhouse where the christening took place, as well as the surrounding 26 acres.

Winslow said the authority, a regional agency supported by six Northern Virginia jurisdictions, is determined to keep the farm running much the way it is now. "The whole idea is to serve the integrity of what is there now . . . . One of the primary missions of the authority is the preservation of land," he said.

"We're going to move very cautiously" about opening the park to visitors, Winslow said.

This will be the first working farm in the 15 regional parks run by the authority, and learning how to manage it will require some breaking in, he acknowledged.

"They'll catch on eventually, just as I had to," Symington said.