The explorers gaped and squinted at the Old Executive Office Building, a rococo heap of colonnades and chimney pots.

"It has 550 rooms and 10 acres of floor space," said their leader, Pearl Oxorn. "It's one of those Victorian anomalies that FDR really would've liked to have seen torn down."

A few blocks farther along, they ogled the Old Post Office, a towering example of Romanesque Revival. "FDR didn't want to tear this down," Oxorn said. "What he wanted to do, actually, was move it to a museum, so everybody would know about 'the horrors of the Elegant Eighties.' "

Like his Nazi b.ete noire, Franklin Delano Roosevelt fancied himself an architect. Because he was president, many folks indulged him, though a few raised Cain. He played master politician, but also master planner, fiddling with blueprints, scratching out sketches and pushing his own designs.

He was supremely confident in his own abilities and never doubted his eye. In 1938, though grappling with greater worries, he promoted his plan for a hospital in Bethesda to Frederic Delano, chairman of the National Capital Park and Planning Commission.

"I have very carefully studied hospital design," FDR wrote Delano, who was also his Uncle Fred, "and, frankly, I am fed up with the type the Government has been building during these past twenty years. . . Therefore, I personally designed a new Naval Hospital with a large central tower of sufficient square footage and height to make it an integral and interesting part of the hospital itself. . ."

To which his shocked Uncle Fred replied: "OH SIRE! . . . since the beginning of time the formula has been that 'the King can do no wrong.' However, from the time of Solomon and even further back, the King found it necessary to surround himself with soothsayers, astrologers, and other wise men to warn him of the pitfalls and dangers lying ahead of him."

As any visitor to Bethesda can see, FDR prevailed. That exchange plus other little- known stories come from art historian William Rhoads; his paper on the subject will be published in the Columbia Historical Society's journal, Records.

The other day Oxorn, an art critic hereabouts, led some neophytes through the Washington that Roosevelt wrought, plus parts of town he tried and failed to fashion to his tastes. The two-mile trek, an "Architectural Outing," was one of several the National Building Museum and the National Museum of American Art will sponsor in coming weeks to mark FDR's centennial.

"He was interested in neoclassicism, or the 'government international' style, because he thought it was the appropriate image for the Federal City," Oxorn told her charges, architects and historians among them. "Another frustrated architect, Adolf Hitler, also liked that style. But fascist government architecture always wanted to intimidate people with super-dimensionality; FDR was willing to pull back."

They were standing, in the morning sun, before one of FDR's favorites: the massive marble-faced Federal Reserve Building at 20th Street and Constitution Avenue NW.

"It was done by a Frenchman, Paul Cret, and finished in 1937," Oxorn said. "FDR had a great liking for Cret, maybe because he also was handicapped. He was deaf from his service in the First World War, and he also suffered from a cancer that prevented him from speaking."

With a sweep of her arm, she added, "You can see there's a unifying whiteness and formal gardens, and a stripped-down elegance of formal style. There's an emphasis on volume and planar surfaces, but also dentals around the top -- sort of stripped-down Beaux-Art ornamentation. FDR called the Fed one of the foremost architectural achievements of Washington."

Oxorn then tackled the building across the street, the annex of the Organization of American States, called the Pan American Union in FDR's time. When the powerful Commission of Fine Arts endorsed the plans, curmudgeonly Harold Ickes persuaded his boss to challenge them. "Ickes thought it would destroy his lovely view of the Mall from the Interior Department," Oxorn said. "And FDR called it 'the new comfort station on the corner.' "

The president dispatched to the august commissioners his own clever design: an open colonnade with rolling lawn dotted with statues of Latin American heroes. The commissioners blanched and tactfully demurred. As things turned out, though, FDR saved Ickes' vista at least for a while; the annex wasn't built until after both men died.

His passion for architecture often sparked equal passion from professional architects. In 1939, as Rhoads reports, he "collaborated" with Henry Toombs on a Dutch Colonial cottage at Hyde Park, and let Life magazine print the plans with the legend, "Franklin D. Roosevelt, Architect." Indiana architect John Lloyd Wright, son of Frank, was quick to pounce. "After seeing the title Architect after F.D. Roosevelt in your magazine, " he wrote to Life, "I give up. Put me in a concentration camp." FDR, though, believed that he had ample precedent in Thomas Jefferson, who also practiced without a license.

Oxorn led her group past the limestone- faced Interior Department, with architect Waddy Wood's six projecting wings, and then to the Old EOB -- about which FDR wrote in 1917, when it was the State, War and Navy Building and he was the Navy's assistant secretary, "It has always been a pet plan of mine to get this building to conform to the general scheme of the Treasury."

Then it was on to the White House, where FDR tinkered with the West Wing, putting in a swimming pool and Oval Office while dreaming fondly of a museum for the East Gallery; and then the Ellipse, from which the Jefferson Memorial's Roman pantheon, designed by John Russell Pope, loomed in the middle distance. "If the tree is in the way, we will move the tree and the lady and the chains, and transplant them to some other place," FDR vowed in 1938, when a group of cherry-tree-lovers threatened to chain themselves to the flora.

He could get quite worked up about his architecture, as suggested by an incident in 1938. A devotee of the Dutch Colonial style in upstate New York, he followed every detail of an emerging post office in Rhinebeck, near his Hyde Park home. One day Henry Morgenthau Jr., secretary of the treasury, placed an urgent call to the supervising architect, Louis Simon. A transcript appears in Rhoads' article on FDR and Dutch Colonial Architecture, published in New York History:

MORGENTHAU: The President of the United States is very much disturbed because he hears that the Rhinebeck Post Office -- they're not going to use old stone wall, that they're planning to open up some stone quarries. And his instructions are that they should use old stone wall.

SIMON: Well, Mr. Secretary, here's what the situation is. We sent Stanley-Brown up there and he found that we could get quite a little stone from the old building that --

MORGENTHAU: Well, you better write me a memo on it. And the President wants old stone wall . . . Well, are you going to use old stone walls?

SIMON: Sure we are.

MORGENTHAU: Well, for God's sake do, please.

SIMON: Yes, we certainly are.

MORGENTHAU: And no new stone.

SIMON: No.

MORGENTHAU: Well, the country is saved. . . Oxorn and company ended their journey on the Federal Triangle, for which FDR approved sites, browbeat planners and presided over construction for such buildings as the Federal Trade Commission, the National Archives, the Justice Department and, not far away, the National Gallery of Art.

Though he took his architecture seriously, he could also take it whimsically, as another story from William Rhoads confirms. In 1942, amid howls from the press and Congress over his sense of esthetics, FDR erected -- on a commanding spot between the Commerce Department and the Willard Hotel -- a huge, colonnaded tn Unioemporary building for the U.S. Information Center. The building finished, FDR wrote Lowell Mellett, director of the Office of Government Reports:

I have but one final suggestion to complete the perfection of this Temple of that ancient god MEL-ET, and that is that we acquire from Messrs. Ringling Brothers an historic supreme Calliope to be put on top of the information sign in order that it may discourse sweet harmony capable of being heard in the Halls of Congress. I suggest also that a special loud speaker from said Calliope be inserted into the offices of The Washington Post. That scheme, luckily, never came to pass.

ARCHITECTURAL OUTINGS -- Free FDR tours, rain or shine, are scheduled for this Saturday and throughout the spring. Call the National Museum of American Art at 357-3095 for details.