The Sisters of Mercy of the Union have sold their national headquarters on 346 acres of lush, rolling Potomac hill country to the U.S. Postal Service for $6.8 million. It will be used as a management training facility.

The order said the proceeds will be used for a variety of projects to provide shelters for homeless women. The order has rented much more modest quarters in Silver Spring.

Included in the sale is the St. Maurice School, which the order operated for 21 years to serve emotionally and mentally handicapped youngsters, and the rambling, five-story headquarters building.

Officials of the order decided more than a year ago to sell the property, which has been its headquarters for half a century, because it wasn't being used enough and in the opinion of many in the order, it was far too opulent for a religious order committed to a life of service.

"It has served the community well," said Sr. Theresa Kane, president of the order, "but the times compel us to adapt to a more simple life and to less affluent ways of expressing ourselves and our ministry."

She added that the contemporary demand for "the church to be in the world is also a call for the religious community to be in the world." For the order to retain ownership of the cloister, located in what has become one of the richest neighborhoods in the country, "was becoming a concern to many sisters," she said.

The Sisters of Mercy of the Union, with 4,700 members, originally specialized in work in schools and hospitals. With the changes in religious outlook brought by the Second Vatican Council, the women have increasingly moved to ministries more directly related to issues of social justice.

Kane, who was overwhelmingly reelected president of the order last spring, was plunged into heated controversy a year ago when she appealed to Pope John Paul II during his visit here to reconsider the church's limitations on leadership roles for women in the Catholic Church.

When the order took over the old 1,000-acre Kendall estate in 1930, the area was largely farmland. Over the years the order has sold off parts of the original estate, including the 18-room mansion, which is now a monastery of a Franciscan order. Another parcel became a subdivision know as McCauley Drive, in which streets are named for pioneers in the religious order.

The new generalate or headquarters building was completed in 1959. Within five years, massive exodus from religious orders was under way and convents emptied all across the land.

For 21 years the order operated St. Maurice School and developed programs that have become models for other facilities for the emotionally handicapped. But changes in federal and state programs for students with learning disabilities have eliminated the need for the private facility, a spokeswoman for the order said, and the school was closed earlier this year.

The Postal Service will use the facility for its management training program and expects to spend $28.1 million for completion of training facilities for the agency, a spokeswoman said. The Postal Service training unit is presently operating in rented quarters in Bethesda.

The additional expenditure will be used to develop a conference center and more housing, said Jeanne O'neill of the Postal Service.

O'neill said that "there will be no mail processing facility" at the Potomac site and that the property will be well maintained by the new owners. "I would think that the residents in the surrounding area will be delighted," she said. CAPTION: Picture 1, Headquarters building and grounds of Roman Catholic nuns' order in Potomac, Md., will become postal training facility; Picture 2, Statue of the Blessed Vrigin looks out over 346-acre, $6.8-million property. Photos by Tom Allen -- The Washington Post