THERE'S A NEW SIGN at Metro's Yellow Line stop at 8th Street and Pennsylvania Avenue. It reads, "The Declaration of Independence, the Constitution and the Bill of Rights are here. At the National Archives." The sign will be unveiled to commemorate the establishment of the National Archives as an independent agency, effective today.

When Congress set up the National Archives 50 years ago, one of its first jobs was to fumigate and catalogue War Department records that had been moldering in a White House garage. The Archives has since developed into a major records repository and research complex including seven presidential libraries and 11 field branches around the country.

The Archives' main building on Pennsylvania Avenue houses the country's most treasured founding documents, provides facilities for historical and genealogical researchers and offers exhibits. The 50th anniversary exhibit now there includes President Reagan playing the part of a pilot in a World War II training film and a recent letter in which he explains how he deals with stress.

The Archives itself suffered stress as it struggled to catch up with 150 years worth of neglected documents and to keep abreast of the geometrically increasing volume of contemporary records. While it was buried within the General Services Administration, it was a poor relative to the well- endowed Library of Congress and Smithsonian Institution. Now, as an independent agency of equal stature, it will be in a better position to attract skilled archivists and fight for the funds it needs.

There is still work to be done to build the Archives into a first-class institution. This depends heavily on who is selected to replace the current Archivist, Robert M. Warner, who is retiring this month. The position requires someone with impeccable credentials in the field and practical organizational and Washington experience. The law creating the new agency specifies that professional qualifications -- not political affiliation -- should be the sole basis for choosing the Archivist. That's an instruction that the president, with his strong interest in preserving the proud reminders of this country's history, will surely keep in mind.