To step into the PSFS room in the William Lescaze exhibition at the Octagon Museum is to experience at once a surge of admiration. The 50-plus-year-old skyscraper -- the initials stand for Philadelphia Saving Fund Society -- was the great triumph of Lescaze's long architectural career in the United States and, still standing, it remains one of the extraordinary monuments of its time.

One's reaction is not primarily nostalgic, though there is that, of course. When Lescaze, with George Howe, designed the building, its interiors and furnishings in the late 1920s and early 1930s, he was riding as surely as he ever would on a wave of the Zeitgeist. The first International Style skyscraper to be built in the United States, this icon of Modern Architecture recalls the boundless optimism of the movement's early years, a time when, despite the bleak economic picture, architects and designers were wont to think of the present as a prelude to an ever-brightening future.

But it is the totality of the design, the way in which everything goes with everything else, that makes a lasting impression. Every window, every stairwell, every desk and every desk set that went into the PSFS building was carefully designed and wrought with its particular place in mind. Few and far between are the 20th-century commercial or corporate structures -- one thinks, for instance, of Frank Lloyd Wright's 1903 Larkin Company building in Buffalo (destroyed in 1950), or his later buildings for Johnson Wax in Racine, Wis. -- in which architects have been able to match details with spaces to such effect.

As a panelist observed in a recent discussion at Syracuse University, where the Lescaze papers are deposited and where this exhibition originated, when the PSFS building was being designed "the whole conceptualization of what that space was . . . was so different from what had happened before that to simply take available furnishings and put them into that space seemed an impossibility. That building seemed to demand the invention of the fittings, the furniture, the lighting fixtures and the clocks by the architects."

The exhibition is admirably calculated to catch this effect, and the intensity of the creative effort that produced it. The PSFS room contains massing studies from which emerged the distinctive final form, with the 32-story ribbon-window office slab set perpendicular to the elevator core and cantilevered over the curved corner base. It also contains floor plans, so that one can tell how the spaces (particularly the two-story-high banking room on the second floor) relate to each other, and numerous objects (including a typical black-painted wood desk with its glistening metal desk set), so that one can experience something of the design's actuality.

Washington is an interesting vantage point from which to observe Lescaze's development, for he designed two buildings here (the Longfellow office building in 1939-'40 and the Swiss Embassy chancery in 1959) that illustrate, respectively, some of the principal failures of the modern movement as it matured in America into a full-fledged commercial vernacular style, and a number of its more enticing aspects.

Lescaze (1896-1969), born and educated in Switzerland, did all of his major work in the United States. He emigrated to this country in search of architectural opportunity after World War I, following a suggestion, he later recalled, from a professor who had asked rhetorically, "Where are you ever going to find the chance of doing monumental work? Egypt? It's too late. Maybe in America."

During the 1920s Lescaze worked on a variety of residential and small-scale commercial projects, educating himself on the job, as it were, in the lexicon of avant-garde architecture, with the result that when monumental opportunity knocked, in the form of the collaboration with Howe on the PSFS commission, he was ready. It is interesting to see that the popular Art Moderne interiors Lescaze was designing for stores and homes in this decade are very close in spirit to those of the PSFS, which represents a very rich, "impure" version of the International Style. One could argue, though perhaps none too vehemently,that Lescaze's "total design" and his Swiss sense of craft in fact produced a species of ornament at a time when ideologues of the International Style were declaring ornament to be, definitively, dead.

The thoroughness of Lescaze's approach to architecture is much in evidence in the exhibition. His own town house studio-home in Manhattan (1933-34), with its striking glass block panels, its fluid floor plan and its built-in furniture and fixtures, is a harmonious work of art in all respects save the way it relates to its older neighbors. For CBS he designed not only a sculptural broadcasting studio in Hollywood (1938) but also microphone cases, broadcasting booths, corporate logos, neon signs and (in 1948), a broadcasting studio on wheels for use in covering the presidential conventions.

Unfortunately for Washington, the Longfellow building (on the northeast corner of Connecticut and Rhode Island avenues NW), though historically important as the city's first modern office structure, is no PSFS. Built for a speculative instead of a corporate office client, it exhibits very little of the thoroughgoing finesse of the Philadelphia building. Doubling the misfortune, the building was subjected to a recent "renovation" that destroyed much of its original appeal -- the new glass is too dark, the new windows lack the subtle refinement of the old, and the entrance is covered over with a tacky-looking metal casing.

Even so, my objections to the Longfellow have less to do with its quality as a building -- those sculptural balconies facing Connecticut Avenue help to make it an attractive period piece -- than with its role as the prototype for the low-cost, flat-roofed, ribbon-windowed, anti-historical, fill-the-zoning-envelope speculative office structure that is, to this day, the bane of downtown D.C. It is hard not to hold Lescaze and his early Modernist colleagues at least partly responsible for this unhappy result. The idea was, as it says in the title of a 1938 magazine article Lescaze wrote, to create "A New Architecture for a Changed World," and we've seen in Washington, in New York and in countless cities across the land just how damaging this fixation on the new has been.

The Swiss chancery, on the southwest corner of Cathedral Avenue and 29th Street NW, is another matter. The modest jewel of his late career, this is altogether a polite, pleasant, modern building: Its L-shaped plan, with parking tucked away on the sides and underneath, is perfect for the function and the site; its low lines blend harmoniously with the trees and sloping hillside, and its see-through entryway is an appropriately diplomatic gesture.

Inside, there are many touches of Lescaze's old genius with the details: The office windows, rising to the full height of the monastery-like office cells, open beautifully to the natural surroundings, and the main stairwell, riserless with granite slab steps, is one of the best of its kind in the city.

The Lescaze exhibition continues at the Octagon (18th Street and New York Avenue NW) through Sept. 29. Coincidentally, there is in town a concurrent exhibition of furnishings and architectural drawings by Charles Rennie Mackintosh (1868-1928), the brilliant turn-of-the-century Scottish architect whose concern with total design preceded that of Lescaze and his fellow Modernists. This superb show at the Federal Reserve Board (C Street between 20th and 21st streets NW) will remain on view through Aug. 30.