Near the spot where Pickett Street comes to a dead end in Alexandria stands a plain brick warehouse with cameras mounted high on the walls.

But the armed guards and locked doors don't protect a hoard of gold bullion or the secrets of the CIA. Instead, they bar an inquisitive public from the documentary history of Richard M. Nixon's presidency.

Inside, 20 experts from the National Archives are working their way through the tapes and documents that the Watergate special prosecutor ordered seized from the White House files of the president and his aides a decade ago. The difficult -- and sometimes subjective -- job has cost the government an estimated $5 million since the Archives began working on Nixon materials in 1971.

The collection includes 40 million pages of documents, three million feet of movie film and videotapes and 350,000 official White House photographs.

Nearby, a vault holds 950 carefully packaged gifts from heads of state and 30,000 other presents given to Nixon, his family and senior White House aides. The collection also includes 4,000 hours of tapes of White House officials' conversations; by comparison, in 1974, the Watergate special prosecutor's office ordered the release of only 12 1/2 hours of tapes.

The archivists' challenge is to get the collection ready for public inspection. Part of it was to have been opened for research this summer, but last fall, 29 former Nixon aides succeeded in challenging the rules that the Archives had drafted to govern the use of the collection.

The Archives' parent agency, the General Services Administration, and the Justice Department are now trying to draw up new rules.

Shortly after leaving office, Nixon tried to gain permanent control of the papers by signing an agreement with then-GSA administrator Arthur Sampson. But Congress voided the document by passing a law that gave the Archives custody of the documents, and required the collection to be protected and the Archives staff to give first priority to organizing the Watergate material.

The first materials that will be made available will include those used by the Watergate special prosecutors, including the files of Nixon, White House chief of staff H.R. Haldeman, domestic policy adviser John D. Ehrlichman and White House counsel John Dean.

At the same time, White House files labeled "outer space," "education," "peace," "American Indians" and "recreation policy" are also expected to be available. Project director James J. Hastings said those files were attacked first because they were among the thinnest, and gave the Archives staff a chance to acquaint itself with the White House filing system.

"When the papers are finally opened, the heavy demand for materials from the special Watergate files is expected to be so great that most of the staff will have to work on research requests," Hastings said. But he has been unsuccessful in getting additional staff or funds. "There are other priorities within the Archives, apparently."

Because the Nixon papers were seized, Hastings said, the Archives has "a responsibility to withdraw anything that is personal that aides or Mr. Nixon would have had a chance to purge from the files before turning them over to the government."

So far, about 50,000 pages -- ranging from the divorce papers of White House aides to personal photographs to insurance policies -- have been returned to the people involved, he said.

"In order for us to decide that a paper is not personal it has to have a direct connection with a presidential-constitutional duty," Hastings said. "For example, if an aide wrote to another aide that 'Sen. Jones is a great guy and we should support him for reelection,' that's personal; if the letter said, 'Sen. Jones is a great guy who supports our Vietnam policy and we should support him for reelection,' we would say that's a presidential document."

The tapes present different kinds of problems. For example, recordings made in the Old Executive Office Building have a lot of background noise from faulty air conditioning and heating units. Recordings of conversations in the Oval Office are often hard to decipher because the microphones were located in the leg well of Nixon's desk and in a lamp, Hastings said.

"Only the telephone tapes are very good because they were direct recordings," he said.

The archivists are using sophisticated equipment to isolate and erase the background noise so future listeners will be able to understand conversations. All of the work is being done on "master duplicates," with the originals locked away in vaults.

The tape review also involves eliminating personal materials, which the archivists remove simply by snipping away sections of tape. The archivists discuss whether the contents are personal and then, with the approval of supervisor Frederick Graboske, make the cuts. The excised portions are being patched together and eventually will be sent to Nixon for his use.

Future listeners will be able to follow guides that will note, for instance, when Mrs. Nixon entered the Oval Office to speak to her husband. A 10-second buzz will signify that personal material has been eliminated.

Hastings said, however, that the Archives has "no intention of doing a transcript."

"It is improper and unethical for an archivist to interpret a document," he said. "With all the checking and double-checking involved, it takes about 250 hours to transcribe one hour of tape and you end up with a questionable document. The first 80 percent of a tape is easy, but the last 20 percent takes a long, long time to get right.