In 1898 John Hay called it "Mr. Mullett's masterpiece."

Hay's friend Henry Adams later recollected it as Mullett's "architectural infant asylum."

In 1958 Harry Truman, pleading an ambivalent case to save the building, fondly referred to it as "the greatest monstrosity in America."

In 1971, which coincidentally was the centennial anniversary of the beginning of its construction, the building and its site were listed on the National Register of Historic Places. Leading architectural historians now regularly refer to it as one of the nation's leading extant examples of the Second Empire Style.

It is, of course, the Old Executive Office Building, that extraordinary pile of columns -- 900 columns! -- and gabled windows -- 1,572 windows! -- overlooking the White House at 17th Street and Pennsylvania Avenue NW, designed by Alfred B. Mullett originally to house the State, War and Navy departments.

No building in Washington, with the possible exception of the recently revived Old Post Office Building, has more directly reflected the swings of the pendulum of architectural taste, and none has excited more vehement animosity or admiration. Small wonder: The building is as unmistakably brash as it is big; like the Old Post Office, it breaks with this city's neoclassical mode; it speaks a language of boosterish opulence and resurgent nationalism typical of the post-Civil War years; it epitomizes a vigorous style, sometimes fittingly called the "General Grant Style," that went out of fashion less than 20 years after it was introduced. Its bold mansard roofs are the quintessential type that Charles Addams satirized for years in his New Yorker cartoons; today these rooftops are celebrated on television, with tongue only partially in cheek, as symbols of power and materialism in such programs as "Falcon Crest."

Obscured by much of the long-lasting argument, which focused mainly on the building's size and style, is the fact that its granite walls enclose a number of the more beautiful spaces and elaborately ornamented rooms in all of Washington. These marvelous places, which before World War II were open to tourists, have been off-limits to the general public ever since, but -- wonder of wonders in an increasingly security-conscious city -- this policy is about to change.

Starting next Saturday at 10 a.m., and continuing Wednesdays (at noon) and Saturdays thereafter, guided tours of some of the building's more outstanding (and less security sensitive) interior spaces will be conducted on an appointment-only basis.

This enlightened, courageous decision was initiated early this year by Christopher Hicks, assistant to the president for management and administration, following the lead of his predecessor in the post, John F.W. Rogers. "We feel that it's a responsibility to open buildings of such historic importance to the public," Hicks explains, "even though we don't get anything out of it other than headaches."

The headaches to which he refers involve security checks on persons who call for tour appointments, and guards to accompany the tours. But the benefits of the program are by no means fanciful, nor solely esthetic. Much that is important and interest- ing in U.S. history took place inside this building. Theodore Roosevelt, William Howard Taft, Franklin Delano Roosevelt and Dwight D. Eisenhower worked there before becoming president; it housed the offices of 25 secretaries of state and 21 secretaries of war; Winston Churchill walked its columned halls; Japanese emissaries had their tense meeting there with Secretary of State Cordell Hull after the bombing of Pearl Harbor; President Richard Nixon squirreled himself away there during Watergate.

Visitors to the building thus will acquire much knowledge of history, and even more tellingly something of its "feel," for the hallways, stairwells and public rooms are not standard, government-issue stuff. To the contrary, they are fine, noble spaces where the activities of government, its immense and serious responsibilities, are given fitting tribute by the art of architecture. The symbolism of making such spaces available, under necessarily controlled circumstances, is immensely important in the capital city of an open society.

Rogers, who has since moved to the Treasury Department as an assistant secretary, has begun to take the same careful kind of look at that historic building as he did at the Old Executive Office Building four years ago. Such architectural awareness in the federal bureaucracy is rare. It is also, in this case, ironic. More than once in its troubled history the State, War and Navy Building was threatened with remodeling along the lines of Robert Mills' Treasury Building, its neoclassical neighbor on the other side of the White House. In 1930 Congress actually authorized $3 million for such a scheme, which fortunately died with the Depression.

In 1981 Rogers hired Mina E. Wright, an architectural historian fresh out of George Washington University, to do research on the building and write a booklet on it (published last year with the unapologetic title, "The Old Executive Office Building: A Victorian Masterpiece," and available from the Government Printing Office for $3.50). Together Rogers and Wright worked out a plan to rehabilitate the building's public spaces gradually, utilizing normal maintenance funds and crews from the General Services Administration.

As a result, visitors will be able to see spaces such as the former State Department Library, now the White House Library and Research Center, a great four-story room surrounded by cast-iron balustrades; the former Navy Library Reception Room, now known as the Indian Treaty Room (no one knows why), another superb chamber whose embellishments, from a floor of Minton tiles to a ceiling with gilded stars, are even richer; and the former War Department Library, now the White House Law Library, where the elaborate fittings are made of cast iron electroplated to look like brass, bronze and black iron. (Another important room, the former office of the secretary of the Navy which is occupied today by the vice president, is understandably off limits.)

Together these notable rooms comprise one of the nation's more exceptional collections of Victorian-era public interiors, but in a way the more impressive facets of the building's interior are the hallways themselves, with their rhythm of cast-iron columns and high mahogany doors with handsome original doorknobs still in place, and the magnificent cantilevered stairwells -- eight in all, each forming a wonderful swooping space more than 80 feet high. At the turn of the century, when the more than 4,000 ornate bronze balusters lining these stairwells were polished regularly, a scrubwoman is said to have remarked that the golden stairs and marble halls looked "just like heaven."

Mullett, who committed suicide after an unsuccessful suit against the government claiming he was underpaid for his labors, deserves full credit for the plan of the building, its formidable massing and richly textured exterior, much of the ornamentation in the south (formerly State Department) wing, and those great stairwells. But two other heroes have re-emerged in recent research: Richard von Ezdorf and Lt. Col. Thomas L. Casey.

Von Ezdorf, an immigrant architect who brought with him memories of baroque palaces in Venice and Vienna, spent 13 years (from 1875, after Mullett's resignation, to 1888, when the building was completed) designing the interiors and ornaments for the east, north and west wings of the building. He labored obscurely and died obscurely, in 1926, but he is being honored today by the restoration of his work.

Casey, the outstanding Army Corps of Engineers officer known for his prominent role in forcing the completion of the Washington Monument to its brilliant conclusion, oversaw the construction of the State, War and Navy Building during its last 11 years. In Casey, thoroughness was combined with inventiveness. He engineered manufacturing systems, particularly in stonecutting, that greatly reduced costs at no apparent sacrifice in quality: The north wing, identical to the south wing but much later in construction, cost 43 percent less!

"Fifty years from now, people may want to tear this building down again," says historian Wright. "But by then," she adds, "enough people will have been inside to want to save it again." Her worries, one hopes, are exaggerated. In the meantime, for tour reservations call 395-5895.