The discovery that scores of historical documents are apparently missing from the National Archives and the Library of Congress exposes a major vulnerability in the government's system for securing some of the nation's written treasures: Anyone using the precious documents can hide them on the body and make off with them.

Archival and library officials acknowledge this, saying that the Constitution bars them from conducting routine body searches or frisking manuscript users as they leave the premises.

One of the nation's leading specialists in search and seizure law said, however, that is not necessarily so. Such searches, like searches at airports, might be constititutional if administered evenhandedly to everyone using the rare documents, said Yale Kamisar of the University of Michigan Law School.

The recent arrests of art historian Charles Merrill Mount and the FBI's discovery of more than 200 historical documents in his possession have raised questions about the security of archival and library materials. And the magnitude of the suspected thefts has forced officials to reexamine the strict but, they acknowledge, obviously not foolproof methods for safeguarding the millions of pieces of paper in their charge.

Any scholar or researcher with credentials is allowed to examine most of the historical manuscripts under supervision at the Archives and the Library of Congress -- including fragile, yellowing letters, diaries and other papers dating back to the founding of the nation. The National Archives houses 3 billion items stored in 195 stack areas. The Library of Congress houses 10,000 collections totaling about 40 million items.

"Our purpose here is to assist scholars and to try to make the collections as accessible as possible within the bounds of security," said James H. Hutson, chief of the manuscript division at the Library of Congress. "That's the balance we have to strike."

Mount, accused of possessing stolen government and other historical documents, was a well-known regular in the research reading rooms of the National Archives and the Library of Congress. He chatted with staff members, occasionally had lunch with a few of them and used a special work space and file cabinets at the Library of Congress.

But for all his familiarity with the hallowed halls of stored history, officials say, about the only way the dapper, British-accented Brooklynite could have removed anything from their midst would have been to conceal the documents under the buttoned vests he was fond of wearing -- the one place library police and archives guards say they could not have searched without probable cause.

Mount, a 59-year-old author and portraitist, has denied to a friend that any of these items belonged to either the National Archives or the Library of Congress, and establishing ownership is expected to be a central element in any future trial. He was released yesterday on $50,000 unsecured bond, with U.S. Magistrate Jean F. Dwyer ordering him to appear for a preliminary hearing on Aug. 28.

Officials at both institutions, meanwhile, say the inability to conduct routine body searches on people leaving the buildings is the only flaw in otherwise elaborate security precautions used to protect their collections. They contend Mount was subject to the same security procedures as any other researcher -- procedures designed to make historical documents and manuscripts available to reputable scholars while protecting the valuable material from theft or damage.

To begin with, officials say, no researcher is given access to the nation's most treasured documents. Presidential papers, including George Washington's diary, and anything having to do with the Continental Congress are microfilmed, and the originals squirreled away in stacks, all of which are locked. Only certain employes are allowed in the stacks.

But much of the other material is not microfilmed, and the sheer volume of documents makes it impossible to catalogue every piece of paper individually, according to officials.

At the National Archives, for example, there are about 3 billion items stored in 195 stack areas. The Library of Congress houses 10,000 collections totaling about 40 million items.

Both institutions file most items in boxes according to subject matter, person and chronology. A researcher usually consults a general inventory of items and then asks to see a particular box or boxes of materials. These are brought to the manuscript reading room, and it is here that officials have set up most of their security.

The archives and library each have lockers where reference material users must check their belongings, including purses and briefcases, before entering the reading room. For taking notes, researchers are given special paper or cards, with holes drilled in one corner. When a researcher leaves the room, it is not unusual for security personnel to slip a pencil through the holed paper -- and shake it -- to make sure no unauthorized documents have been hidden between the papers.

The archives will allow some researchers to bring in a few pieces of paper with their notes, but these are stamped for identification purposes beforehand. The library prohibits all outside materials. Neither institution has hidden cameras, though installing such equipment has been considered in the past, but each has security personnel who watch the reading room and observe all those who enter or leave.

"We have our staff on elevated platforms so they can have a better view of the room," said James B. Byers, chief of the reference services branch at the archives.

When a researcher has finished reviewing material -- it could be a portion of pension records dating back to the Revolutionary War or anthropologist Margaret Mead's notebooks -- they are returned to the central reference desk. The original request slip is cross-checked against the material, and the time of checkout and return is noted.

Researchers are permitted to copy some materials; those deemed too bulky or valuable are copied by staff. All copies are marked as coming from either the library or archives.

Hutson, noting that about 20 percent of the library's collection is on microfilm, said guarding and preserving the materials are equal goals. The more valuable the collection or item, the more restrictions on its availability.

"Obviously," he said, "we're going to treat Jefferson and Lincoln a little differently than, with all due respect, {Woodrow Wilson's secretary of state} Robert Lansing."